Steel pipe excellent in formability and method for producing thereof

ABSTRACT

The present invention is a high strength steel pipe excellent in formability in hydroforming and similar forming methods, characterized by: containing, in mass, C of 0.0005 to 0.30%, Si of 0.001 to 2.0%, Mn of 0.01 to 3.0% and appropriate amounts of other elements if necessary, with the balance consisting of Fe and unavoidable impurities; and an average for the ratios of the X-ray strength in the orientation component group of {110}&lt;110&gt; to {111}&lt;110&gt; to random X-ray diffraction strength on a plane at the wall thickness center being 2.0 or more and/or a ratio of the X-ray strength in the orientation component of {110}&lt;110&gt; to random X-ray diffraction strength on the plane at the wall thickness center being 3.0 or more.

TECHNICAL FILED

[0001] The present invention relates to a steel material used for, forexample, undercarriage components, structural members, etc. of anautomobile or the like and, in particular, a high strength steel pipeexcellent in formability in hydroforming or the like, and to a method ofproducing the same.

BACKGROUND ART

[0002] The strengthening of a steel sheet has been desired with thegrowing demands for weight reduction in automobiles. The strengtheningof a steel sheet makes it possible to reduce the weight of an automobilethrough the reduction of material thickness and also to improvecollision safety. Attempts have been made recently to form a materialsteel sheet or pipe of a high strength steel into components ofcomplicated shapes by the hydroforming method for the purpose ofreducing the number of components or welded flanges, in response to thedemands for the weight reduction and cost reduction of an automobile.Actual application of new forming technologies, such as the hydroformingmethod (see Japanese Unexamined Patent Publication No. H10-175027), isexpected to bring about great advantages such as the reduction of costsand increase in the degree of freedom in design work.

[0003] In order to fully enjoy the advantages of the hydroformingmethod, new materials suitable for the new forming methods are required.For instance, the influence of r-value on the hydroforming work wasdisclosed at the 50^(th) Japanese Joint Conference for the Technology ofPlasticity (in 1999, p. 447 of its proceedings). What was disclosed was,however, that, based on an analysis by a simulation, the r-value in thelongitudinal direction was effective for T-shape forming, which was oneof the fundamental forming modes of hydroforming. Apart from the above,as reported at FISITA World Automotive Congress, 2000A420 (Jun. 12-15,2000, at Seoul), a high formability steel pipe was being developedaiming at realizing high strength and high ductility by forming finecrystal grains. The improvement of the r-value in the longitudinaldirection of a steel pipe was also discussed in the report.

[0004] However, while the formation of fine crystal grains is veryeffective for securing ductility of thick materials, considering thepoints that, according to the report, fine crystal grains are obtainedby warm working at comparatively low temperatures and that a heavy draft(the ratio of diameter reduction or area reduction, in this case) isapplied during the working, it is possible that the reported methodlowers the n-value, which is important for the forming by hydroformingand similar methods, and does not increase average r-value, which is anindicator of formability.

[0005] As reviewed above, there are very few cases of practicaldevelopments of materials suitable not only for a certain basic formingmode such as hydroforming or the like but also for various formingmodes. Thus, in the absence of suitable materials, conventional highr-value steel sheets and high ductility steel sheets are used for thehydroforming applications.

DISCLOSURE OF THE INVENTION

[0006] The present invention provides a steel pipe excellent informability in hydroforming and similar forming methods and a method ofproducing the steel pipe by specifying the characteristics of the steelmaterial for the pipe.

[0007] The present inventors identified the metallographic structure andtexture of a steel material excellent in formability in hydroforming andsimilar forming methods and a method for controlling the metallographicstructure and texture. On this basis, the present invention provides asteel pipe excellent in formability in hydroforming and similar formingmethods and a method of producing the steel pipe, by specifying thestructure and texture and the method for controlling them.

[0008] The gist of the present invention, therefore, is as follows:

[0009] (1) A steel pipe excellent in formability characterized by:containing, in mass,

[0010] C: 0.0005 to 0.30%,

[0011] Si: 0.001 to 2.0%,

[0012] Mn: 0.01 to 3.0%,

[0013] with the balance consisting of Fe and unavoidable impurities; andthe average for the ratios of the X-ray strength in the orientationcomponent group of {110}<110> to {111}<110> to random X-ray diffractionstrength on a plane at the wall thickness center being 2.0 or moreand/or the ratio of the X-ray strength in the orientation component of{110}<110> to random X-ray diffraction strength on a plane at the wallthickness center being 3.0 or more.

[0014] (2) A steel pipe excellent in formability according to the item(1), characterized by further containing, in the steel, one or more ofAl, Zr and Mg at 0.0001 to 0.5 mass % in total.

[0015] (3) A steel pipe excellent in formability according to the item(1) or (2), characterized by further containing, in the steel, one ormore of Ti, V and Nb at 0.001 to 0.5 mass % in total.

[0016] (4) A steel pipe excellent in formability according to any one ofthe items (1) to (3), characterized by further containing P at 0.001 to0.20 mass % in the steel.

[0017] (5) A steel pipe excellent in formability according to any one ofthe items (1) to (4), characterized by further containing B at 0.0001 to0.01 mass % in the steel.

[0018] (6) A steel pipe excellent in formability according to any one ofthe items (1) to (5), characterized by further containing in the steelone or more of Cr, Cu, Ni, Co, W and Mo at 0.001 to 1.5 mass % in total.

[0019] (7) A steel pipe excellent in formability according to any one ofthe items (1) to (6), characterized by further containing in the steelone or more of Ca and a rare earth element (Rem) at 0.0001 to 0.5 mass %in total.

[0020] (8) A steel pipe excellent in formability according to any one ofthe items (1) to (7), characterized in that: ferrite accounts for 50% ormore, in terms of area percentage, of the metallographic structure; thecrystal grain size of the ferrite is within the range from 0.1 to 200μm; and the average for the ratios of the X-ray strength in theorientation component group of {110}<110> to {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center is 2.0 ormore and/or the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center is 3.0 or more.

[0021] (9) A steel pipe excellent in formability characterized bysatisfying either one or both of the following properties:

[0022] (1) the n-value in the longitudinal direction of the pipe being0.12 or more, and

[0023] (2) the n-value in the circumferential direction of the pipebeing 0.12 or more.

[0024] (10) A steel pipe excellent in formability according to the item(9), characterized by the property of the r-value in the longitudinaldirection of the pipe being 1.1 or more.

[0025] (11) A steel pipe excellent in formability characterized in thatthe texture of the steel pipe satisfies one or more of the followingconditions {circle over (1)} to {circle over (3)}:

[0026] {circle over (1)} at least one or more of the following ratiosbeing 3.0 or more: the ratio of the X-ray strength in the orientationcomponent of {111}<110> to random X-ray diffraction strength on a planeat the wall thickness center; the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {332}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center,

[0027] {circle over (2)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and

[0028] {circle over (3)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {111}<110> to {111}<112>and {554}<225> to random X-ray diffraction strength on a plane at thewall thickness center being 2.0 or more; and the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center being 3.0or more.

[0029] (12) A steel pipe excellent in formability according to any oneof the items (9) to (11), characterized by containing ferrite at 50% ormore in terms of area percentage and the grain size of the ferrite beingin the range from 0.1 to 200 μm.

[0030] (13) A steel pipe excellent in formability according to any oneof the items (9) to (12), characterized by: containing ferrite at 50% ormore in terms of area percentage; the grain size of the ferrite rangingfrom 1 to 200 μm; and the standard deviation of the distribution of thegrain size falling within the range of ±40% of the average grain size.

[0031] (14) A steel pipe excellent in formability according to any oneof the items (9) to (13), characterized by: containing ferrite by 50% ormore in terms of area percentage; and the average for the aspect ratios(the ratio of the grain length in the longitudinal direction to thegrain thickness in the thickness direction) of ferrite grains being inthe range from 0.5 to 3.0.

[0032] (15) A steel pipe excellent in formability according to any oneof the items (9) to (14), characterized by containing, in mass,

[0033] C: 0.0005 to 0.30%,

[0034] Si: 0.001 to 2.0%,

[0035] Mn: 0.01 to 3.0%,

[0036] P: 0.001 to 0.20%, and

[0037] N: 0.0001 to 0.03%,

[0038] with the balance consisting of Fe and unavoidable impurities.

[0039] (16) A steel pipe excellent in formability according to the item(15), characterized by further containing in the steel, in mass, one ormore of

[0040] Ti: 0.001 to 0.5%,

[0041] Zr: 0.001 to 0.5% or less,

[0042] Hf: 0.001 to 2.0% or less,

[0043] Cr: 0.001 to 1.5% or less,

[0044] Mo: 0.001 to 1.5% or less,

[0045] W: 0.001 to 1.5% or less,

[0046] V: 0.001 to 0.5% or less,

[0047] Nb: 0.001 to 0.5% or less,

[0048] Ta: 0.001 to 2.0% or less, and

[0049] Co: 0.001 to 1.5% or less.

[0050] (17) A steel pipe excellent in formability according to the item(15) or (16), characterized by further containing in the steel, in mass,one or more of

[0051] B: 0.0001 to 0.01%,

[0052] Ni 0.001 to 1.5%, and

[0053] Cu: 0.001 to 1.5%.

[0054] (18) A steel pipe excellent in formability according to any oneof the items (15) to (17), characterized by further containing in thesteel, in mass, one or more of

[0055] Al: 0.001 to 0.5%,

[0056] Ca: 0.0001 to 0.5%,

[0057] Mg: 0.0001 to 0.5%, and

[0058] Rem: 0.0001 to 0.5%.

[0059] (19) A method of producing a steel pipe excellent in formabilityaccording to any one of the items (1) to (18), characterized by forminga mother pipe using a hot-rolled or cold-rolled steel sheet satisfyingany one or more of the following conditions {circle over (1)} to {circleover (4)} as the material sheet, then heating the mother pipe to atemperature in the range from the Ac₃ transformation point to 200° C.above the Ac₃ transformation point, and then subjecting it to diameterreduction work in the temperature range from 900 to 650° C.:

[0060] {circle over (1)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 2.0 or more; and the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 3.0 or more,

[0061] {circle over (2)} at least one or more of the following ratiosbeing 3.0 or more: the ratio of the X-ray strength in the orientationcomponent of {111}<110> to random X-ray diffraction strength on a planeat the wall thickness center; the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {332}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center,

[0062] {circle over (3)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and

[0063] {circle over (4)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {111}<110> to {111}<112>and {554}<225> to random X-ray diffraction strength on a plane at thewall thickness center being 2.0 or more; and the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center being 3.0or more.

[0064] (20) A method of producing a steel pipe excellent in formabilityaccording to any one of the items (1) to (18), characterized by forminga mother pipe using a hot-rolled or cold-rolled steel sheet satisfyingany one or more of the following conditions {circle over (1)} to {circleover (4)} as the material sheet, and then applying heat treatment to themother pipe at a temperature in the range from 650° C. to 200° C. abovethe Ac₃ transformation point:

[0065] {circle over (1)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 2.0 or more; and the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 3.0 or more,

[0066] {circle over (2)} at least one or more of the following ratiosbeing 3.0 or more: the ratio of the X-ray strength in the orientationcomponent of {111}<110> to random X-ray diffraction strength on a planeat the wall thickness center; the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {332}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center,

[0067] {circle over (3)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and

[0068] (4) at least either one or both of the following conditions beingsatisfied: the average for the ratios of the X-ray strength in theorientation component group of {111}<110> to {111}<112> and {554}<225>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 2.0 or more; and the ratio of the X-ray strength in theorientation component of {111}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 1.5 or more.

[0069] (21) A steel pipe excellent in formability characterized bysatisfying either one or both of the following properties:

[0070] (1) the n-value in the longitudinal direction of the pipe being0.18 or more, and

[0071] (2) the n-value in the circumferential direction of the pipebeing 0.18 or more.

[0072] (22) A steel pipe excellent in formability according to the item(21), characterized by having the property of the r-value in thelongitudinal direction of the pipe being 0.6 or more but less than 2.2.

[0073] (23) A steel pipe excellent in formability according to the item(21) or (22), characterized in that the ratio of X-ray strength torandom X-ray diffraction strength satisfies the following twoconditions:

[0074] {circle over (1)} the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 1.5 or more, and

[0075] {circle over (2)} the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 5.0 or less.

[0076] (24) A steel pipe excellent in formability according to any oneof the items (21) to (23), characterized in that the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center is 3.0 ormore.

[0077] (25) A steel pipe excellent in formability according to any oneof the items (21) to (24), characterized by containing ferrite by 50% ormore in terms of area percentage and the grain size of the ferrite beingin the range from 0.1 to 200 μm.

[0078] (26) A steel pipe excellent in formability according to any oneof the items (21) to (25), characterized by: containing ferrite by 50%or more in terms of area percentage; and the average for the aspectratios (the ratio of the grain length in the longitudinal direction tothe grain thickness in the thickness direction) of ferrite grains beingin the range from 0.5 to 3.0.

[0079] (27) A steel pipe excellent in formability according to any oneof the items (21) to (26), characterized by containing, in mass,

[0080] C: 0.0005 to 0.30%,

[0081] Si: 0.001 to 2.0%,

[0082] Mn: 0.01 to 3.0%, and

[0083] N: 0.0001 to 0.03%,

[0084] with the balance consisting of Fe and unavoidable impurities.

[0085] (28) A steel pipe excellent in formability according to any oneof the items (21) to (27), characterized by further containing in thesteel pipe one or more of Al, Zr and Mg at 0.0001 to 0.5 mass % intotal.

[0086] (29) A steel pipe excellent in formability according to any oneof the items (21) to (28), characterized by further containing in thesteel pipe one or more of Ti, V and Nb at 0.001 to 0.5 mass % in total.

[0087] (30) A steel pipe excellent in formability according to any oneof the items (21) to (29), characterized by further containing P at0.001 to 0.20 mass % in the steel pipe.

[0088] (31) A steel pipe excellent in formability according to any oneof the items (21) to (30), characterized by further containing B at0.0001 to 0.01 mass % in the steel pipe.

[0089] (32) A steel pipe excellent in formability according to any oneof the items (21) to (31), characterized by further containing in thesteel pipe one or more of Cr, Cu, Ni, Co, W and Mo at 0.001 to 5.0 mass% in total.

[0090] (33) A steel pipe excellent in formability according to any oneof the items (21) to (32), characterized by further containing in thesteel pipe one or more of Ca and a rare earth element (Rem) at 0.0001 to0.5 mass % in total.

[0091] (34) A method of producing a steel pipe excellent in formabilityaccording to any one of the items (21) to (33), characterized by forminga mother pipe, then heating it to a temperature in the range from 50° C.below the Ac₃ transformation point to 200° C. above the Ac₃transformation point, and then subjecting it to diameter reduction workin the temperature range from 650 to 900° C. at a diameter reductionratio of 10 to 40%.

BEST MODE FOR CARRYING OUT THE INVENTION

[0092] The present invention is explained hereafter in detail. Theinvention according to the item (1) is explained in the first place.

[0093] The contents of elements in the explanations below are in masspercentage.

[0094] C: C is effective for increasing steel strength and, hence,0.0005% or more of C is added but, since an addition of C in a largequantity is undesirable for controlling steel texture, the upper limitof its addition is set at 0.30%.

[0095] Si: Si is an element for increasing strength and deoxidizingsteel as well and, therefore, its lower limit is set at 0.001%. Anexcessive addition of Si, however, leads to the deterioration ofwettability in plating and workability and, for this reason, the upperlimit of the Si content is set at 2.0%.

[0096] Mn is an element effective for increasing steel strength andtherefore the lower limit of its content is set at 0.01%. The upperlimit of the Mn content is set at 3.0%, because its excessive additionlowers ductility.

[0097] The ratios of X-ray strength in orientation component group of{110}<110> to {111}<110> and orientation component of {110}<110> torandom X-ray diffraction strength on plane at a wall thickness centerconstitute the property figures most strongly required in theapplication of hydroforming. The average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength, which ratios being obtained by anX-ray diffraction measurement on a plane at the wall thickness center,is determined to be 2.0 or more.

[0098] The main orientations included in this orientation componentgroup are {110}<110>, {661}<110>, {441}<110>, {331}<110>, {221}<110>,{332}<110>, {443}<110>, {554}<110> and {111}<110>.

[0099] The ratios of the X-ray strength in these orientations to randomX-ray diffraction strength can be calculated from the three-dimensionaltexture calculated by the vector method based on the pole figure of{110}, or the three-dimensional texture calculated by the seriesexpansion method based on two or more pole figures of {110}, {100},{211} and {310}.

[0100] For example, in case of obtaining the ratios of the X-raystrength in the crystal orientation components to random X-raydiffraction strength by the latter method, the ratios can be representedby the strengths of (110)[1-10], (661)[1-10], (441)[1-10], (331)[1-10],(221)[1-10], (332)[1-10], (443)[1-10], (554)[1-10] and (111)[1-10] at aφ₂=45° cross section in the three-dimensional texture.

[0101] The average for the ratios of the X-ray strength in theorientation component group of {110}<110> to {111}<110> to random X-raydiffraction strength means the arithmetic average for the ratios of theX-ray strength in the above orientation components to random X-raydiffraction strength. When the X-ray strengths in not all the aboveorientation components are obtained, the arithmetic average of the X-raystrengths of the orientation components of {110}<110>, {441}<110> and{221}<110> may be used as a substitute. Among these orientationcomponents, {110}<110> is important and it is particularly desirablethat the ratio of the X-ray strength in this orientation component torandom X-ray diffraction strength be 3.0 or more. Needless to say, it isbetter yet, especially for a steel pipe for hydroforming use, if theaverage for the ratios of X-ray strength in the orientation componentgroup of {110}<110> to {111}<110> to random X-ray diffraction strengthis 2.0 or more and, at the same time, the ratio of X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthis 3.0 or more.

[0102] Further, in the case where the shape of a product requires acomparatively large amount of axial compression in a mode of formingwork, it is desirable that the average for the ratios of the X-raystrength in the above orientation group to random X-ray diffractionstrength be 3.5 or more and the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthbe 5.0 or more.

[0103] In the invention according to the item (11), it is necessary thatthe texture of the steel pipe satisfies one or more of the followingconditions {circle over (1)} to {circle over (3)}:

[0104] {circle over (1)} at least one or more of the following ratiosbeing 3.0 or more: the ratio of the X-ray strength in the orientationcomponent of {111}<110> to random X-ray diffraction strength on a planeat the wall thickness center; the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {332}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center,

[0105] {circle over (2)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and

[0106] {circle over (3)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {111}<110> to {111}<112>and {554}<225> to random X-ray diffraction strength on a plane at thewall thickness center being 2.0 or more; and the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center being 3.0or more.

[0107] Regarding the limitation of the X-ray strengths in theorientation components in the condition (1), even if the orientationcomponent of {111}<110> among the orientation component group of{110}<110> to {111}<110> is omitted from the arithmetic average, theeffects of the present invention are retained.

[0108] That is to say, the high formability (a diameter expansion ratioof 1.25 or more under different hydroforming conditions) intended in thepresent invention can be achieved if at least one or more of thefollowing ratios is/are 3.0 or more, on a plane at the wall thicknesscenter: the ratio of the X-ray strength in the orientation component of{111}<110> to random X-ray diffraction strength; the average for theratios of the X-ray strength in the orientation component group of{110}<110> to {332}<110> to random X-ray diffraction strength; and theratio of the X-ray strength in the orientation component of {110}<110>to random X-ray diffraction strength.

[0109] As described above, at least the ratios of the X-ray strength inthe orientation component group of {110}<110> to {332}<110> and theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center are important characteristicfigures for forming by the hydroforming method.

[0110] Regarding the limitation of the X-ray strengths in theorientation components in the condition (2), when at least the averagefor the ratios of the X-ray strength in the orientation component groupof {100}<110> to {223}<110> to random X-ray diffraction strength on aplane at the wall thickness center exceeds 3.0, or at least the ratio ofthe X-ray strength in the orientation component of {100}<110> to randomX-ray diffraction strength on a plane at the wall thickness centerexceeds 3.0, the diameter expansion ratio or the like particularly inhydroforming, which is an object of the present invention, deterioratesto about 1.2 or less. For this reason, the value of each of the above islimited to 3.0 or less.

[0111] Regarding the limitation of the X-ray strengths in theorientation components in condition (3), when the average for the ratiosof the X-ray strength in the orientation component group of {111}<110>to {111}<112> and {554}<225> to random X-ray diffraction strength on aplane at the wall thickness center is below 2.0 or the ratio of theX-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength on a plane at the wall thickness center isbelow 3.0, the diameter expansion ratio in hydroforming also tends tobecome low. For this reason, it is necessary to secure the degrees ofconvergence of 2.0 or more and 3.0 or more, respectively, in the above.Thus, together with the conditions {circle over (1)} and {circle over(2)}, it is necessary to satisfy at least one or more of the conditions{circle over (1)} to {circle over (3)} for securing the formability inhydroforming.

[0112] The ratios of the X-ray strength in the above orientationcomponents are measured by X-ray diffraction measurement on a plane atthe wall thickness center and calculating the ratios of X-ray strengthin the orientation components to the X-ray diffraction strength of arandom crystal.

[0113] The main orientation components included in the above orientationcomponent groups are explained below.

[0114] The main orientation components included in the orientationcomponent group of {110}<110> to {332}<110> are {110}<110>, {661}<110>,{441}<110>, {331}<110>, {221}<110>, {332}<110>, {443}<110> and{554}<110>.

[0115] The main orientation components included in the orientationcomponent group of {100}<110> to {223}<110> are {100}<110>, {116}<110>,{114}<110>, {113}<110>, {112}<110>, {335}<110> and {223}<110>.

[0116] The main orientation components included in the orientationcomponent group of {111}<110> to {111}<112> are {111}<110> and{111}<112>.

[0117] The ratios of the X-ray strength in these orientation componentsto random X-ray diffraction strength can be calculated from thethree-dimensional texture calculated by the vector method based on thepole figure of {110}, or the three-dimensional texture calculated by theseries expansion method based on two or more pole figures of {110},{100}, {211} and {310}.

[0118] For example, the ratios of the X-ray strength in the orientationcomponents included in the orientation component group of {110}<110> to{332}<110> to random X-ray diffraction strength can be calculated by thelatter method from the strengths of (110)[1-10], (661)[1-10],(441)[1-10], (331)[1-10], (221)[1-10], (332)[1-10], (443)[1-10] and(554)[1-10] at a φ₂=45° cross section in the three-dimensional texture.Likewise, in the case of the orientation component group of {100}<110>to {223}<110>, the strengths of (001)[1-10], (116)[1-10], (114)[1-10],(113)[1-10], (112)[1-10], (335)[1-10] and (223)[1-10] can be used asrepresentative figures and, in the case of the orientation componentgroup of {111}<110> to {111}<112>, the strengths of (111)[1-10] and(111)[-1-12] can be used as representative figures.

[0119] In addition, when it is impossible to obtain the X-ray strengthfor all the above orientation components included in the orientationcomponent group of {110}<110> to {332}<110>, which is of specialimportance for the purpose of the present invention, an arithmeticaverage in the strengths of the orientation components of (110)[1-10],(441)[1-10] and (221)[1-10] can be used as a substitute.

[0120] Note that the X-ray strength of the texture of the steel pipeaccording to the present invention usually becomes the strongest in therange of the above orientation component group at the φ₂=45° crosssection and, the farther away from the above orientation component groupthe orientation component is, the lower the strength level thereofgradually becomes. Considering the factors such as the accuracy in X-raymeasurement, axial twist during the pipe production, and the accuracy inthe X-ray sample preparation, however, there may be cases where theorientation in which the X-ray strength is the strongest deviates fromthe above orientation component group by about ±5° to ±10°.

[0121] For the X-ray diffraction measurement of a steel pipe, arcsection test pieces have to be cut out from the steel pipe and pressedinto flat pieces for X-ray analysis. Further, when pressing the arcsection test pieces into flat pieces, the strain must be as low aspossible to avoid the influence of crystal rotation caused by theworking and, for this reason, the upper limit of the amount of imposedstrain is set at 10%, and the working has to be done under a strain notexceeding the figure. Then, the tabular test pieces thus prepared areground to a prescribed thickness by mechanical polishing and thenconditioned by a chemical or other polishing method so as to remove thestrain and expose the thickness center layer for the X-ray diffractionmeasurement.

[0122] Note that, when a segregation band is found in the wall thicknesscenter layer, the measurement may be done at an area free fromsegregation anywhere in the range from ⅜ to ⅝ of the wall thickness.Further, even when no segregation band is found, it is acceptable forthe purpose of the present invention if a texture specified in claims ofthe present invention is obtained at a plane other than the plane at thewall thickness center and, for instance, in the above range from ⅜ to ⅝of the wall thickness. Additionally, when the X-ray diffractionmeasurement is difficult, the EBSP or ECP technique may be employed forthe measurement.

[0123] Although the texture of the present invention is specified interms of the result of the X-ray measurement at a plane at the wallthickness center or near it as stated above, it is preferable that thesteel pipe have a similar texture also in wall thickness portions otherthan near the thickness center. However, there may be cases where thetexture in the range from the outer surface to ¼ or so of the wallthickness does not satisfy the requirements described above, because thetexture changes as a result of shear deformation during the diameterreduction work explained hereafter.

[0124] Note that {hkl}<uvw> means that, when the test pieces for theX-ray diffraction measurement are prepared in the manner describedabove, the crystal orientation perpendicular to the wall surface is<hkl> and the crystal orientation along the longitudinal direction ofthe steel pipe is <uvw>.

[0125] The characteristics of the texture according to the presentinvention cannot be expressed using common inverse pole figures andconventional pole figures only, but it is preferable that the ratios ofthe X-ray strength in the above orientation components to random X-raydiffraction strength be as specified below when, for example, theinverse pole figures expressing the radial orientations of the steelpipe are measured at portions near the wall thickness center: 2 or lessin <100>, 2 or less in <411>, 4 or less in <211>, 15 or less in <111>,15 or less in <332>, 20.0 or less in <221> and 30.0 or less in <110>.

[0126] In the inverse pole figures expressing the axial orientation, thepreferred figures of X-ray strength ratios are as follows: 10 or more inthe <110> orientation and 3 or less in all the orientations other thanthe <110> orientation.

[0127] Then, the invention according to the item (9) is explainedhereafter.

[0128] N-value: It is sometimes the case in hydroforming that working isapplied to a work piece isotropically to some extent and, accordingly,it is necessary to secure the n-value in the longitudinal and/orcircumferential directions of the steel pipe. For this reason, the lowerlimit of n-value is set at 0.12 for both the directions. The effects ofthe present invention are realized without setting an upper limit ofn-value specifically.

[0129] In the present invention, n-value is defined as the valueobtained at an amount of strain of 5 to 10% or 3 to 8% in the tensiletest method according to Japanese Industrial Standard (JIS).

[0130] Next, the invention according to the item (10) is explainedhereafter.

[0131] R-value: Since hydroforming includes working with material influxthrough the application of axial compression and, hence, for securingworkability at the portions subjected to this kind of working, the lowerlimit of the r-value in the longitudinal direction of a steel pipe isset at 1.1. The effects of the present invention are realized withoutsetting an upper limit of r-value specifically.

[0132] In the present invention, r-value is defined as the valueobtained at an amount of strain of 10% or 5% in the tensile testaccording to JIS.

[0133] The reasons for limiting the chemical composition in theinvention according to the items (2) to (7) and (15) to (18) areexplained hereafter.

[0134] Al, Zr and Mg: These are deoxidizing elements. Among these, Alcontributes to the enhancement of formability especially when boxannealing is employed. An excessive addition of these elements causesthe crystallization and precipitation of oxides, sulfides and nitridesin quantities, deteriorating steel cleanliness and ductility. Besides,it remarkably spoils a plating property. For this reason, it isdetermined to add one or more of these elements if necessary, at 0.0001to 0.50% in total, or within the limits of 0.0001 to 0.5% for Al, 0.0001to 0.5% for Zr and 0.0001 to 0.5% for Mg.

[0135] Nb, Ti and V: Any of Nb, Ti and V, which are added if necessary,increases steel strength by forming carbides, nitrides or carbonitrideswhen added at 0.001% or more, either singly or in total of two or moreof them. When their total content or the content of any one of themexceeds 0.5%, they precipitate in great quantities in the grains offerrite, which is the base phase, or at the grain boundaries in the formof carbides, nitrides or carbonitrides, deteriorating ductility. Theaddition range of Nb, Ti and V is, therefore, limited to at 0.001 to0.5% in single addition or in total of two or more of them.

[0136] P: P is an element effective for enhancing steel strength, but itdeteriorates weldability and resistance to delayed crack of slabs aswell as fatigue resistance and ductility. For this reason, P isdetermined to be added only when necessary and the range of its additionis limited to at 0.001 to 0.20%.

[0137] B: B, which is added if necessary, is effective for strengtheninggrain boundaries and increasing steel strength. When its addition amountexceeds 0.01%, however, the above effect is saturated and, what is more,steel strength is increased more than necessary and workability isdeteriorated in addition. For this reason, the content of B is limitedto at 0.0001 to 0.01%.

[0138] Ni, Cr, Cu, Co, Mo and W: These are steel hardening elements andtherefore 0.001% or more of these elements is added, if necessary,either singly or in total of two or more of them. Since an excessiveaddition of these elements lowers ductility, their addition range islimited to at 0.001 to 1.5% in a single addition or in a total of two ormore of them.

[0139] Ca and a rare earth element (Rem): They are elements effectivefor the control of inclusions, and their addition in an appropriateamount increases hot workability. Their excessive addition, however,causes hot shortness, and thus the range of their addition is defined asat 0.0001 to 0.5% in single addition or in total of two or more of them,as required. Here, the rare earth elements (Rems) include Y, Sr and thelanthanoids. Industrially, it is economical to add these elements in theform of mischmetal, which is a mixture of them.

[0140] N: N is effective for increasing steel strength and it may beadded at 0.0001% or more. Its addition in a large quantity is, however,not desirable for the control of welding defects and, for this reason,the upper limit of its addition amount is set at 0.03%.

[0141] Hf and Ta: Hf and Ta, which are added if necessary, increasesteel strength through the formation of carbides, nitrides orcarbonitrides when added at 0.001% or more each. When added in excess of2.0%, however, they precipitate in quantities in the grains of ferrite,which is the base phase, or at the grain boundaries in the form of thecarbides, nitrides or carbonitrides, deteriorating ductility. Theaddition range of Hf and Ta, therefore, is defined as at 0.001 to 2.0%each.

[0142] The effects of the present invention are not hindered even whenO, Sn, S, Zn, Pb, As, Sb, etc. are included in the steel pipe asunavoidable impurities as long as each addition amount is within therange of at 0.01% or less.

[0143] Crystal grain size: The control of crystal grain size isimportant for controlling texture. It is necessary for intensifying theX-ray strength in the orientation component of {110}<110>, particularlyin the invention according to the items (8) to (12), to control thegrain size of main phase ferrite to 0.1 to 200 μm. The orientationcomponent of {110}<110> is most important for enhancing formability inthe orientation component group of {110}<110> to {332}<110>. Thus, evenif the grain size of ferrite is mixed in a wide range, for example in ametallographic structure in which the portions consisting of ferritegrains 0.1 to 10 μm in size and those consisting of ferrite grains 10 to100 μm in size exist in a mixture, the effects of the present inventionare maintained as long as a high X-ray strength is obtained in theorientation component of {110}<110>. Here, the ferrite grain size ismeasured by the section method compliant to JIS.

[0144] By the way, for measuring the size and the aspect ratio offerrite grains, it is necessary to make grain boundaries clearlyidentifiable. Ferrite grain boundaries can be clearly identified byusing a 2 to 5% nitral solution in the case of steels having acomparatively high carbon content, or a special etching solution,SULC-G, in the case of ultra-low carbon steels (such as IF steels),after finishing a section surface, for observation, with polishingdiamond having a roughness of several micrometers or by buffing.

[0145] The special etching solution can be prepared by dissolving 2 to10 g of dodecylbenzenesulfonic acid, 0.1 to 1 g of oxalic acid and 1 to5 g of picric acid in 100 ml of water and then adding 2 to 3 ml of 6Nhydrochloric acid. In the structure obtained through the abovetechniques, ferrite grain boundaries appear and their sub-grains alsomay appear partially.

[0146] The ferrite grain boundaries meant here are the interfacesrendered visible to a light-optical microscope by the above samplepreparation processes, including the interfaces such as the sub-grainsappearing partially. The size and aspect ratio of ferrite grains aremeasured with respect to the grain boundaries thus observed. The ferritegrains are measured through image analysis of 20 or more fields of viewof 100 to 500-power magnification, and the grain size, aspect ratio,etc. are calculated on the basis of this measurement. The areapercentage of ferrite is measured assuming that the ferrite grains arespherical. Note that the value of area percentage is nearly equal tothat of volume percentage.

[0147] The material of the steel pipe according to the present inventionmay also contain structures such as pearlite, bainite, martensite,austenite, carbonitrides, etc. as metallographic structures other thanferrite. For the purpose of securing steel ductility, however, thepercentage of these hard phases is limited to below 50%. The range ofthe grain size of ferrite is determined to be from 0.1 to 200 μm,because it is industrially difficult to obtain recrystallization grainssmaller than 0.1 μm in size, and, when crystal grains larger than 200 μmare mixed, the X-ray strength in the orientation component of {110}<110>falls.

[0148] In the invention according to the items (13) and (14), inaddition, the standard deviation of the grain size of ferrite grains andtheir aspect ratio are limited for the purpose of increasing the ratioof X-ray strength in the orientation component group of {110}<110> to{332}<110> and suppressing the ratio of X-ray strength in theorientation component group of {100}<110> to {223}<110>.

[0149] These figures are calculated through the observation of 20 ormore fields of view by a light-optical microscope of 100 to 1,000-powermagnification, and the standard deviation of the grain size iscalculated based on the circle-equivalent diameters of the grainsobtained by image analysis.

[0150] The aspect ratio is calculated from the ratio of the number ofthe ferrite grain boundaries crossing a line segment parallel to thedirection of rolling to the number of the ferrite grain boundariescrossing a line segment of the same length perpendicular to thedirection of rolling and from the following equation: aspect ratio=(thenumber of grain boundaries crossing the line segment perpendicular tothe rolling direction)/(the number of grain boundaries crossing the linesegment parallel to the rolling direction). When the standard deviationof the ferrite grain size exceeds ±40% of the average grain size, or theaspect ratio is over 3 or below 0.5, formability tends to deteriorate.For this reason, the above figures are defined as the upper and lowerlimits of respective items.

[0151] In the invention according to the item (13), the lower limit ofthe ferrite grain size is set at 1 μm for the purpose of raising theratios of the X-ray strength in the orientation component of {111}<110>and/or the orientation component group of {111}<110> to {332}<110>.

[0152] In producing the steel pipe according to the present invention,steel is refined in a blast furnace or an electric arc furnace process,then subjected to various secondary refining processes and,subsequently, cast by an ingot casting or a continuous casting method.In the case of continuous casting, if a production process such as theone to hot-roll cast slabs without cooling is employed in combinationwith other production processes, the effects of the present inventionare not hindered in the least.

[0153] In addition to the above, the effects of the present inventionare not in the least adversely affected if the following productionprocesses are combined in the production of the steel sheets for pipeforming: heating an ingot to a temperature from 1,050 to 1,300° C. andthen hot-rolling it at a temperature in the range from not lower than10° C. below the Ar₃ transformation point to lower than 120° C. abovethe Ar₃ transformation point; the application of roll lubrication duringhot rolling; coiling a hot band at a temperature of 750° C. or below;the application of cold rolling; and the application of box annealing orcontinuous annealing after cold rolling. That is to say, a hot-rolled,cold-rolled or cold-rolled and annealed steel sheet may be used as thematerial steel sheet for the pipe forming.

[0154] Besides the above, the effects of the present invention areretained even when 0.01% or less of any one of O, Sn, S, Zn, Pb, As, Sb,etc. is mixed in the steel. In pipe forming, electric resistancewelding, TIG welding, MIG welding, laser welding, UO press method, buttwelding and other welding and pipe forming methods may be employed.

[0155] The invention according to the items (19) and (20) (a method ofproducing a steel pipe excellent in formability) will be explainedhereafter.

[0156] The texture of a hot-rolled or cold-rolled steel sheet: It is aprerequisite for improving the formability of a steel pipe to satisfyany one or more of the following conditions {circle over (1)} to {circleover (4)}:

[0157] {circle over (1)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 2.0 or more; and the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 3.0 or more,

[0158] {circle over (2)} at least one or more of the following ratiosbeing 3.0 or more: the ratio of the X-ray strength in the orientationcomponent of {111}<110> to random X-ray diffraction strength on a planeat the wall thickness center; the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {332}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {110}<110> to random X-ray diffraction strength on a plane at thewall thickness center,

[0159] {circle over (3)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and

[0160] {circle over (4)} at least either one or both of the followingconditions being satisfied: the average for the ratios of the X-raystrength in the orientation component group of {111}<110> to {111}<112>and {554}<225> to random X-ray diffraction strength on a plane at thewall thickness center being 2.0 or more; and the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center being 3.0or more.

[0161] Heating temperature: In order to improve the formability of weldjoints, the heating temperature before diameter reduction is set at theAc₃ transformation point or above and, in order to prevent crystalgrains from becoming coarse, the heating temperature is limited to 200°C. above the Ac₃ transformation point or below.

[0162] Temperature of diameter reduction work: In order to facilitatethe recovery from the strain hardening after the diameter reduction, thetemperature during diameter reduction work is set at 650° C. or higherand, in order to prevent crystal grains from becoming coarse, thetemperature is limited to 900° C. or below.

[0163] Temperature of heat treatment after pipe forming: The heattreatment is applied for the purpose of recovering the ductility of asteel pipe lowered by the strain during pipe forming. When thetemperature is below 650° C., a sufficient ductility recovery effect isnot forthcoming, but, when the temperature exceeds 200° C. above the Ac₃transformation point, coarse crystal grains become conspicuous and thesurface quality of the steel pipe is remarkably deteriorated. For thisreason, the temperature is limited in the range from 650° C. to 200° C.above the Ac₃ transformation point.

[0164] In the above production process of welded steel pipe, solutionheat treatment may be applied locally as deemed necessary for obtainingrequired characteristics at the heat affected zones of the welded seam,independently or in combination, and several times repeatedly, ifnecessary. This will enhance the effects of the present invention yetfurther. The heat treatment is meant for the application only to thewelded seam and the heat affected zones, and it can be applied on-lineduring the pipe forming or off-line. The effects of the presentinvention are not in the least hindered if diameter reduction orhomogenizing heat treatment prior to the diameter reduction is appliedto the steel pipe. Further, it is desirable for improving formability toapply lubrication during the diameter reduction process; the lubricationhelps realize the effects of the present invention, as it enables theproduction of a steel pipe excellent in forming workability in which thedegree of convergence of the x-ray strength in the orientation componentof {111}<110> and/or the orientation component group of {110}<110> to{332}<110> is enhanced all across the wall thickness, as a product inwhich the texture, especially in the surface layer, is controlled to theranges specified in the claims of the present invention.

[0165] The invention according to the item (21) will be explainedhereafter.

[0166] The N-value in longitudinal and/or circumferential direction(s)of steel pipe: This is important for enhancing the workability inhydroforming and similar working without causing the breakage orbuckling of a work piece and, for this reason, an n-value is determinedto be 0.18 or more in the longitudinal and/or circumferentialdirection(s). It is often the case that, depending on the mode ofdeformation during forming work, the amount of deformation is uneven inthe longitudinal or circumferential direction. In order to secure goodworkability under different working methods, it is desirable thatn-value be 0.18 or more in the longitudinal and circumferentialdirections.

[0167] In the case of extremely heavy working, it is desirable thatn-value be 0.20 or more in both the longitudinal and circumferentialdirections. The effects of the present invention can be obtained withoutdefining an upper limit of n-value specifically. There are, however,cases that, depending on the process of working, a high r-value isrequired in the longitudinal direction of a steel pipe. In such a case,in consideration of the conditions of diameter reduction work and otherfactors, it may become desirable to control n-value to 0.3 or less andincrease the r-value in the longitudinal direction of the steel pipe.

[0168] The invention according to the item (22) will be explainedhereafter.

[0169] R-value in longitudinal direction of steel pipe: According topast research, such as a report in the 50^(th) Japanese Joint Conferencefor the Technology of Plasticity (in 1999, p. 447 of its proceedings),the influence of r-value on the working by hydroforming was analyzedusing simulations, and the r-value in the longitudinal direction wasfound effective in T-shape forming, one of the fundamental deformationmodes of hydroforming. Besides the above, at the FISITA World AutomotiveCongress, 2000A420 (Jun. 12-15, 2000, at Seoul), it was reported thatthe r-value in the longitudinal direction could be enhanced byincreasing the ratio of diameter reduction.

[0170] Even when the r-value in the longitudinal direction is enhancedby increasing the ratio of diameter reduction, however, if the n-value,another important characteristic figure for formability, is lowered,that does not mean an improvement in the workability of a steel pipe ina practical sense. On the other hand, as the size of work piecesincreased, it became necessary to secure formability, not only in theportions where, like in T-shape forming, hydroforming or similar workingwas done so as to secure a sufficient material influx, but also in theportions where the material influx was comparatively small. In such asituation, the present inventors discovered that, while it was necessaryto maintain a high n-value, it was effective to reduce the ratio ofdiameter reduction or conduct the diameter reduction work at acomparatively high temperature so as to lower the r-value in thelongitudinal direction.

[0171] When the r-value in the longitudinal direction is below 2.2, itbecomes easy to secure a desired level of n-value in the longitudinaland/or circumferential directions) in commercial production and, forthis reason, the upper limit of the r-value is set at 2.2.

[0172] The lower limit of r-value is set at 0.6 or more from theviewpoint of securing formability.

[0173] The invention according to the item (23) is explained hereafter.

[0174] Texture: In order to secure formability, the following twoconditions must be satisfied:

[0175] {circle over (1)} the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter being 1.5 or more; and

[0176] {circle over (2)} the ratio of the X-ray strength in theorientation component of {110}<110> to random X-ray diffraction strengthon a plane at the wall thickness center being 5.0 or less.

[0177] Outside the above ranges, it is possible that n-value maydeteriorate.

[0178] In addition, in order to enhance formability and realize a goodbalance between n-value and r-value, it is desirable that the ratio ofX-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength be 3.0 or more on a plane at the wallthickness center.

[0179] The ratio of the X-ray strength in the orientation component of{111}<110> is important in the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength. It is particularly desirable thatthe ratio of the X-ray strength to random X-ray diffraction strength be3.0 or more in this orientation component, especially when productshaving a complicated shape or a large size are formed.

[0180] Needless to say, when the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength is 2.0 or more and the ratio of theX-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength is 3.0 or more, such a steel pipe is betterstill, especially for hydroforming use.

[0181] The orientation component of {110}<110> is also an importantorientation component. For securing good values of ductility and then-values in the longitudinal and circumferential directions of the steelpipe, however, it is necessary that the ratio of the X-ray strength inthe orientation component of {110}<110> to random X-ray diffractionstrength be 5.0 or less and, for this reason, its upper limit is set at5.0.

[0182] Note that {hkl}<uvw> means that, when the test pieces for theX-ray diffraction measurement are prepared in the manner describedabove, the crystal orientation perpendicular to the wall surface is<hkl> and the crystal orientation along the longitudinal direction ofthe steel pipe is <uvw>.

[0183] The principal orientations included in these orientationcomponents and orientation component groups are the same as thoseexplained in the item (1).

[0184] Crystal grain size and aspect ratio: Since it is difficult toobtain crystal grains smaller than 0.1 μm in size industrially, andformability is adversely affected when there are crystal grains largerthan 200 μm, these figures are defined as the lower and upper limits,respectively, of the grain size, the same as in the invention accordingto the item (12). The range of aspect ratio is defined as explained inthe item (14).

[0185] Next, the reasons for limiting the chemical composition of theinvention according to the item (27) and the successive items areexplained.

[0186] The reasons for limiting the chemical composition are the same asin the section of the invention according to the item (1) explainedbefore.

[0187] In addition to the above, the content of N is specified for thefollowing reason.

[0188] N: N is effective for strengthening steel and thus it is added at0.0001% or more, but since its addition in a large quantity is notdesirable for the control of welding defects, the upper limit of itscontent is set at 0.03%.

[0189] The reasons for limiting the chemical composition of theinvention according to the items (27) to (33) are the same as thoseexplained in relation to the inventions according to the items (2) to(7) and (15) to (18).

[0190] Ni, Cr, Cu, Co, Mo and W: As an excessive addition of theseelements causes the deterioration of ductility, the addition amount ofthese elements is limited to at 0.001 to 5.0% in single addition or intotal of two or more of them.

[0191] Further, the effects of the present invention are not hinderedeven if 0.01% or less of any of O, Sn, S, Zn, Pb, As, Sb, etc. isincluded as an unavoidable impurity.

[0192] Next, the invention according to the item (34) will be explainedhereafter. The reasons for limiting production conditions are the sameas those of the invention according to the item (19) except for thefollowing.

[0193] After being formed, a mother pipe is heated to a temperature from50° C. below the Ac₃ transformation point to 200° C. above the Ac₃transformation point and undergoes diameter reduction work at 650° C. orhigher at a diameter reduction ratio of 40% or less.

[0194] Whereas a heating temperature lower than 50° C. below the Ac₃transformation point causes the deterioration of ductility and theundesirable formation of texture, a heating temperature higher than 200°C. above the Ac₃ transformation point causes the deterioration ofsurface properties owing to oxidation, besides the formation of coarsecrystal grains. For this reason, the heating temperature is limited tothe range specified above.

[0195] In addition, the temperature of the diameter reduction work islimited as described above because, when the temperature is lower than650° C., n-value is lowered. No upper limit is set forth specificallyfor the temperature of the diameter reduction work, but it is desirableto limit it to880° C. or below for fear that the surface properties maydeteriorate owing to oxidation. Besides, when the diameter reductionratio exceeds 40%, the decrease in n-value becomes conspicuous and it isfeared that ductility and surface properties are deteriorated. For thesereasons, the diameter reduction ratio is limited as specified above. Thelower limit of the diameter reduction ratio is set at 10% foraccelerating the formation of texture.

[0196] The diameter reduction ratio is the value obtained by subtractingthe quotient of the outer diameter of a product pipe divided by thediameter of a mother pipe from 1, and it means the amount by which thediameter is reduced through the working.

[0197] It is desirable for improving formability to use lubrication onthe diameter reduction work. The lubrication furthers the effects of thepresent invention, since it makes the texture especially in the surfacelayer conform to the range specified in the present invention, enhancesthe degree of convergence of the X-ray strengths to the orientationcomponent of {111}<110> and/or the orientation component group of{111}<110> to {111}<110> throughout the wall thickness and appropriatelysuppresses the degree of convergence of the X-ray strengths to theorientation component of {110}<110> and, accordingly, makes it possibleto produce a high strength steel pipe excellent in formability byapplying various forming modes of hydroforming and similar formingmethods.

EXAMPLE Example 1

[0198] The steels of the chemical compositions shown in Tables 1 on 4were refined on a laboratory scale, heated to 1,200° C., hot-rolled intosteel sheets 2.2 and 7 mm in thickness at a finish rolling temperaturefrom 10° C. below the Ar₃ transformation point, which is determined bythe chemical composition and cooling rate of steel, to less than 120° C.above the Ar₃ transformation point (roughly 900° C.). Some of the steelsheets thus obtained were used for pipe forming and others for coldrolling.

[0199] Some of the cold-rolled steel sheets were further subjected to anannealing process to obtain cold-rolled and annealed steel sheets 2.2 mmin thickness. Then, the steel sheets were formed, in the cold, intosteel pipes 108 to 49 mm in outer diameter by TIG, laser or electricresistance welding. Thereafter, the steel pipes were heated to atemperature from the Ac₃ transformation point to 200° C. above it andsubjected to diameter reduction work at 900 to 650° C. to obtain highstrength steel pipes 75 to 25 mm in outer diameter.

[0200] Forming work by hydroforming under the condition of an axialcompression amount of 1 mm at 100 bar/mm was applied to the steel pipesfinally obtained until they burst. A scribed circle 10 mm in diameterwas transcribed on each steel pipe beforehand, and the strain εφ in thelongitudinal direction of the pipe and the strain εθ in thecircumferential direction were measured near the fracture or the portionof the maximum wall thickness reduction. Then the diameter expansionratio at which the ratio of the two strains ρ=εφ/εθ was equal to −0.5(the value was negative because the wall thickness decreased) wascalculated, and the diameter expansion ratio was used as an indicator ofthe formability in hydroforming for the evaluation of the product pipes.

[0201] X-ray analysis was carried out on flat test pieces prepared bycutting out arc section test pieces from the steel pipes and thenpressing them. The relative X-ray strength of the test pieces wasobtained through the comparison with the X-ray strength of a randomcrystal. The n-values in the longitudinal and circumferential directionswere measured at a strain amount of 5 to 10% or 3 to 8% and the r-valuesin the above directions at a strain amount of 10 or 5% on arc sectiontest pieces cut out for the respective purposes.

[0202] Tables 1 to 4 show, for each of the steels, the ratios of theX-ray strength in the orientation component of {110}<110> and theorientation component group of {110}<110> to {111}<110> to random X-raydiffraction strength and the diameter expansion ratio (the ratio of thepipe diameter at the portion where the expression ρ=εφ/εθ=−0.5 was trueat the time of bursting to the initial diameter) at which each steelpipe burst during hydroforming.

[0203] Each of invented steels A to U demonstrated a relative X-raystrength in the orientation component of {110}<110> of 3.0 or more, anaverage for the ratios of the X-ray strength in the orientationcomponent group of {110}<110> to {111}<110> to random X-ray diffractionstrength of 2.0 or more and a diameter expansion ratio as good as morethan 1.25.

[0204] The relative X-ray strength in the orientation component of{110}<110> in any of invented steels NA to NG was higher than those ofinvented steels A to U and the diameter expansion ratio was as good asmore than 1.3 in most of them, despite the pipe materials beinghot-rolled steel sheets.

[0205] In contrast, in the comparative steels, namely in high-C steel V,high-Mg steel W, high-Nb steel X, high-B steel Z, high-Mo steel AA andhigh-Rem steel BB, the ratios of the X-ray strength in the orientationcomponent of {110}<110> and the orientation component group of{110}<110> to {111}<110> to random X-ray diffraction strength were lowand the diameter expansion ratio was also low. On the other hand, inhigh-P steel Y, although the relative X-ray strength in the orientationcomponent of {110}<110> was high, the workability of its welded jointwas low and, consequently, the diameter expansion ratio was low.

[0206] Table 5 shows the relation between the area percentages offerrite by grain size range and the diameter expansion ratio of steelsA, B and P. The grain size distribution was measured on specimens forlight-optical microscope observation prepared by etching a sectionsurface parallel to the direction of rolling by the etching methodexplained before and using a dual image processing analyzer. In thesesteels, the structure of which was a mixed grain structure, the X-raystrength in the orientation component of {110}<110> was higher than thatin other orientation components and the diameter expansion ratio wasalso high. TABLE 1 Steel C Si S Mn Al Zr Mg Ti V Nb P B Cr Cu Ni Mo Co WCa Rem A 0.045 0.15 0.006 0.3 A ″ ″ ″ ″ B 0.055 0.6 0.005 0.1 0.0050.005 B ″ ″ ″ ″ ″ ″ B ″ ″ ″ ″ ″ ″ B ″ ″ ″ ″ ″ ″ B ″ ″ ″ ″ ″ ″ C 0.0280.01 0.007 0.3 0.041 0.025 C ″ ″ ″ ″ ″ ″ C ″ ″ ″ ″ ″ ″ C ″ ″ ″ ″ ″ ″ D0.056 0.03 0.006 0.3 0.052 0.12 D ″ ″ ″ ″ ″ ″ D •' ″ ″ ″ ″ ″ D ″ ″ ″ ″ ″″ E 0.002 0.05 0.004 0.4 0.01 0.005 E ″ ″ ″ ″ ″ ″ F 0.036 0.05 0.003 0.20.006 0.0025 F ″ ″ ″ ″ ″ ″ F ″ ″ ″ ″ ″ ″ F ″ ″ ″ ″ ″ ″ G 0.002 0.050.005 0.2 0.04 0.05 0.01 G ″ ″ ″ ″ ″ ″ ″ G ″ ″ ″ ″ ″ ″ ″ G ″ ″ ″ ″ ″ ″ ″Average relative X-ray strength in orientation Relative Heating Seamcomponent X-ray Diameter temperature welding group of strength inexpansion before method {110}<110> orientation ratio at diameter forpipe − component of bursting reduction Steel Forming {111}<110>{110}<110> by HF /° C. A Laser 2.6 4.1 1.3 Invented 770 A steel-cold ALaser 2.5 3.9 1.3 Invented 770 A steel-hot B Laser 2.8 4.2 1.3 Invented770 B steel-cold B ERW 2.7 4.1 1.26 Invented 770 B steel-cold B ERW 2.64.2 1.25 Invented 770 B steel-hot B ERW 5.3 10.5 1.31 Invented 850 Bsteel-cold B ERW 5.2 9.8 1.3 Invented 850 B steel-hot C Laser 2.2 3.91.35 Invented 750 C steel-cold C ERW 2.3 4 1.34 Invented 750 Csteel-cold C TIG 2.3 4 1.38 Invented 750 C steel-cold C TIG 2.3 3.9 1.36Invented 750 C steel-hot D Laser 2.2 3.5 1.27 Invented 700 D steel-coldD ERW 2.2 3.6 1.26 Invented 700 D steel-cold D ERW 4.6 5.6 1.32 Invented840 D steel-hot D ERW 6.3 7.6 1.31 Invented 840 D steel-cold E Laser 2.24 1.27 Invented 700 E steel-cold E Laser 2.1 3.9 1.26 Invented 700 Esteel-hot F Laser 2.3 3.8 1.26 Invented 750 F steel-cold F Laser 2.2 3.71.25 Invented 750 F steel-hot F Laser 4.5 6.3 1.29 Invented 770 Fsteel-hot F Laser 5.1 7 1.28 Invented 770 F steel-cold G Laser 2.6 4.11.37 Invented 700 G steel-cold G Laser 2.3 3.8 1.32 Invented 700 Gsteel-hot G Laser 3.5 5.6 1.35 Invented 835 G steel-cold G Laser 4.5 3.91.34 Invented 835 G steel-hot

[0207] TABLE 2 (continued from Table 1) Steel C Si S Mn Al Zr Mg Ti V NbP B Cr Cu Ni Mo Co W Ca Rem H 0.002 0.07 0.006 0.3 0.046 0.03 0.02 0.01H ″ ″ ″ ″ ″ ″ ″ ″ I 0.02 0.1 0.005 0.2 0.03 0.1 I ″ ″ ″ ″ ″ ″ J 0.0020.05 0.003 0.2 0.035 0.02 0.02 0.02 0.0006 J ″ ″ ″ ″ ″ ″ ″ ″ ″ J ″ ″ ″ ″″ ″ ″ ″ ″ J ″ ″ ″ ″ ″ ″ ″ ″ ″ K 0.023 0.1 0.004 0.2 0.036 0.01 0.2 K ″ ″″ ″ ″ ″ ″ L 0.003 0.05 0.006 0.2 0.038 0.04 0.01 0.2 0.1 L ″ ″ ″ ″ ″ ″ ″″ ″ M 0.002 0.1 0.003 0.3 0.044 0.04 0.015 0.5 M ″ ″ ″ ″ ″ ″ ″ ″ M ″ ″ ″″ ″ ″ ″ M ″ ″ ″ ″ ″ ″ ″ N 0.02 0.09 0.002 0.2 0.06 0.2 O 0.003 0.080.003 0.1 0.05 0.05 0.5 P 0.051 0.6 0.004 0.7 0.036 0.02 0.002 P ″ ″ ″ ″″ ″ ″ P ″ ″ ″ ″ ″ ″ ″ Q 0.048 0.5 0.008 0.6 0.045 0.008 0.0005 Q ″ ″ ″ ″″ ″ ″ R 0.07 0.8 0.006 1.2 0.04 Average relative X-ray strength inorientation Relative Heating Seam component X-ray Diameter temperaturewelding group of strength in expansion before method {110}<110>orientation ratio at diameter for pipe − component of bursting reductionSteel forming {111}<110> {110}<110> by HF /° C. H Laser 2.7 4.3 1.36Invented 750 H steel-cold H Laser 2.5 3.7 1.31 Invented 750 H steel-hotI Laser 2.3 3.6 1.28 Invented 750 I steel-cold I Laser 2.2 3.4 1.26Invented 750 I steel-hot J Laser 2.3 4 1.34 Invented 750 J steel-cold JLaser 2.2 3.6 1.3 Invented 750 J steel-hot J Laser 4.5 8.1 1.32 Invented850 J steel-hot J Laser 6 9.1 1.33 Invented 850 J steel-cold K Laser 2.23.6 1.28 Invented 750 K steel-cold K Laser 2.2 3.5 1.28 Invented 750 Ksteel-hot L Laser 2.3 3.5 1.27 Invented 700 L steel-cold L Laser 2.3 3.61.26 Invented 700 L steel-hot M Laser 2.4 3.9 1.31 Invented 750 Msteel-cold M Laser 2.3 4 1.3 Invented 750 M steel-hot M Laser 7.5 10.11.32 Invented 850 M steel-cold M Laser 6.5 10 1.33 Invented 850 Msteel-hot N Laser 2.6 4.1 1.3 Invented 750 N steel-cold O Laser 2.5 4.21.34 Invented 750 O steel-cold P Laser 2.7 4.5 1.34 Invented 750 Psteel-cold P Laser 5.6 7.5 1.36 Invented 900 P steel-cold P ERW 6.5 8.51.36 Invented 900 P steel-hot Q Laser 2.7 4.2 1.31 Invented 750 Qsteel-cold Q Laser 2.7 4.3 1.31 Invented 750 Q steel-hot R Laser 2.2 3.51.27 Invented 700 R steel-cold

[0208] TABLE 3 (continued from Table 2) Steel C Si S Mn Al Zr Mg Ti V NbP B Cr Cu Ni Mo Co W Ca Rem S 0.002 0.1 0.005 1.1 0.04 0.04 T 0.02 0.10.005 1 0.05 U 0.002 0.1 0.006 0.9 0.03 0.05 0.09 V 0.32 0.3 0.003 10.026 0.01 V ″ ″ ″ ″ ″ ″ V ″ ″ ″ ″ ″ ″ V ″ ″ ″ ″ ″ ″ W 0.025 0.05 0.0030.2 0.008 0.6 W ″ ″ ″ ″ ″ ″ X 0.052 0.6 0.006 0.7 0.032 2.1 0.013 X ″ ″″ ″ ″ ″ ″ Y 0.05 0.1 0.009 0.3 0.045 0.45 Y ″ ″ ″ ″ ″ ″ Y ″ ″ ″ ″ ″ ″ Y″ ″ ″ ″ ″ ″ Average relative X-ray strength in orientation RelativeHeating Seam component X-ray Diameter temperature welding group ofstrength in expansion before method {110}<110> orientation ratio atdiameter for pipe − component of bursting reduction Steel forming{111}<110> {110}<110> by HF /° C. S Laser 2.8 4.1 1.3 Invented 750 Ssteel-cold T Laser 2.3 3.8 1.29 Invented 750 T steel-cold U Laser 2.64.2 1.32 Invented 750 U steel-cold V Laser 0.02 0.05 1.18 Comparative700 V steel-cold: C outside range V ERW 0.02 0.04 1.15 Comparative 700 Vsteel-cold: C outside range V ERW 0.02 0.03 1.14 Comparative 700 Vsteel-hot: C outside range V TIG 0.03 0.05 1.22 Comparative 800 Vsteel-cold: C outside range W Laser 0.05 0.03 1.02 Comparative 770 Wsteel-cold: Mg outside range W Laser 0.04 0.03 1.03 Comparative 770 Wsteel-hot: Mg outside range X Laser 0.03 0.03 1.07 Comparative 770 Xsteel-cold: Nb outside range X Laser 0.02 0.03 1.05 Comparative 770 Xsteel-hot: Nb outside range Y Laser 2.1 3.2 1.05 Comparative 750 Ysteel-cold: P outside range Y ERW 2 3.2 1.1 Comparative 800 Ysteel-cold: P outside range Y TIG 2.1 3.1 1.08 Comparative 750 Ysteel-cold: P outside range Y TIG 2 3 1.12 Comparative 800 Y steel-hot:P outside range

[0209] TABLE 4 (continued from Table 3) Steel C Si S Mn Al Zr Mg Ti V NbP B Cr Cu Ni Mo Co W Ca Rem Z 0.048 0.5 0.008 0.5 0.041 0.03 0.1 Z ″ ″ ″•• ″ ″ ″ AA 0.049 0.5 0.01 0.8 0.023 0.02 2 AA ″ ″ ″ ″ ″ ″ ″ BB 0.0460.5 0.003 0.8 0.033 0.02 0.55 BB ″ ″ ″ ″ ″ ″ ″ NA 0.007 0.01 0.014 0.10.03 NA ″ ″ ″ ″ ″ NB 0.012 0.01 0.005 0.5 0.04 0.011 NB ″ ″ ″ ″ ″ ″ NC0.051 0.01 0.001 0.3 0.05 ND 0.002 0 0.005 0.1 0.031 0.06 0.007 NE 0.0550.02 0.016 0.2 0.044 NF 0.002 0.01 0.005 0.1 0.03 0.02 0.001 NG 0.210.01 0.005 0.1 0.03 Average relative X-ray strength in orientationRelative Heating Seam component X-ray Diameter temperature welding groupof strength in expansion before method {110}<110> orientation ratio atdiameter for pipe − component of bursting reduction Steel forming{111}<110> {110}<110> by HF /° C. Z Laser 0.02 0.05 1.1 Comparative 770Z steel-cold: B outside range Z Laser 0.02 0.06 1.07 Comparative 770 Zsteel-hot: B outside range AA Laser 0.05 0.15 1.12 Comparative 770 AAsteel-cold: Mo outside range AA Laser 0.04 0.1 1.11 Comparative 770 AAsteel-hot: Mo outside range BB Laser 0.04 0.2 1.15 Comparative 770 BBsteel-cold: REM outside range BB Laser 0.03 0.15 1.15 Comparative 770 BBsteel-hot: REM outside range NA Laser 3.1 5.6 1.36 Invented 950 NAsteel-hot NA ERW 5.1 10 1.39 Invented 950 NA steel-hot NB Laser 4.9 8.31.37 Invented 850 NB steel-hot NB ERW 7.1 11.5 1.39 Invented 980 NBsteel-hot NC ERW 6.3 10.5 1.36 Invented 840 NC steel-hot ND ERW 3.9 5.71.34 Invented 840 ND steel-hot NE ERW 4 6.9 1.35 Invented 840 NEsteel-hot NF ERW 3.6 7.5 1.33 Invented 880 NF steel-hot NG ERW 3 6.31.26 Invented 840 NG steel-hot

[0210] TABLE 5 Average relative Area Area X-ray strength in X-raystrength percentage percentage of orientation ratio in of grains grainsover component group of orientation 0.1-10 10-200 μm Diameter {110}<110>− component of Steel μm in size in size expansion ratio {111}<110>{110}<110> A 30 70 1.3 3.5 4.1 B 20 80 1.3 3.7 4.2 P 15  80* 1.34 3.94.5 {111}<110> − {110}<110> − {100}<110> − {111}<112> + Steel {111}<110>{332}<110> {223}<110> {100}<110> {554}<225> A 3 4 0.5 0 1 B 3 4.1 0.5 01 P 3 4.2 0.5 0 1

Example 2

[0211] The steels of the chemical compositions shown in Tables 6 and 7were refined on a laboratory scale, heated to 1,200° C., hot-rolled intosteel sheets 2.2 and 7 mm in thickness at a finish rolling temperaturefrom 10° C. below the Ar₃ transformation point, which is determined bythe chemical composition and cooling rate of the steel, to less than120° C. above the Ar₃ transformation point (roughly 900° C.). Some ofthe steel sheets thus obtained were used for pipe forming and others forcold rolling.

[0212] Some of the cold-rolled steel sheets were further subjected to anannealing process to obtain cold-rolled and annealed steel sheets 2.2 mmin thickness. Then the steel sheets were formed in the cold into steelpipes 108 to 49 mm in outer diameter by electric resistance welding.Thereafter, high strength steel pipes were produced in the followingmanner: heating some of the steel pipes to the temperatures shown inTables 8 and 9 and then subjecting them to diameter reduction work up toan outer diameter of 75 to 25 mm at the temperatures also shown inTables 8 and 9; and subjecting the others to heat treatment after thepipe forming.

[0213] Hydroforming work was applied to the steel pipes finally obtaineduntil they burst. The hydroforming was applied at different amounts ofaxial compression and inner pressure through the control of theseparameters until the pipes burst or buckled. Then, the longitudinalstrain εφ and circumferential strain εθ were measured at the portionshowing the largest diameter expansion ratio (diameter expansionratio=the largest circumference after forming/the circumference ofmother pipe) and the portion near the fracture or the portion of themaximum wall thickness reduction. The ratio of the two strains ρ=εφ/εθand the maximum diameter expansion ratio were plotted, and the diameterexpansion ratio at which the value of εφ/εθ was −0.5 (the value wasnegative as the wall thickness decreased) was calculated. This diameterexpansion ratio was also used for the evaluation of the steel pipes asanother indicator of the formability in hydroforming.

[0214] Tables 8 and 9 also show the characteristics of the steels. Thesteels the matrices of which had the X-ray strength, n-values andr-values falling within the respective ranges specified in the presentinvention demonstrated high diameter expansion ratios. The pipes heatedto above the Ac₃ transformation point for the diameter reduction alsoshowed high diameter expansion ratios. With respect to the areapercentage and grain size distribution of ferrite, most of the steelshad ferrite as the main phase and an average grain size of 100 μm orless. As can be understood from the average grain size and its standarddeviation, the ferrite grains 0.1 μm or less or 200 μm or more in sizewere not seen in them.

[0215] On the other hand, in the cases where the heating temperaturebefore the diameter reduction or the temperature during the diameterreduction work was low (steels NDD, NFF and NJJ), the diameter expansionratio was low. In high-C steel CNNA, high-Nb steel CNBB and high-B steelCNCC, the diameter expansion ratio was also low. Further, in steels CNAAand CNBB, the amount of hard phases was high and their crystal grainsizes could not be measured accurately. TABLE 6 Kind of steel sheet andseam welding Steel C Si Mn P Facultative elements method {111}<110> NAA0.124 0.01 0.41 0.01 0.03Al Hot-rolled, ERW 5.6 NAA ″ ″ ″ ″ ″Hot-rolled, ERW 12 NAA* ″ ″ ″ ″ ″ Hot-rolled, ERW 0.5 NBB 0.08 0.14 0.380.01 0.02Al Hot-rolled, ERW 6 NBB* ″ ″ ″ ″ ″ Hot- rolled, ERW 0.5 NCC0.01 0.01 0.11 0.02 0.04Al Hot-rolled, ERW 8 NCC* ″ ″ ″ ″ ″ Hot-rolled,ERW 1.5 NDD 0.002 0.02 0.95 0.07 0.04Al-0.05Ti Hot-rolled, ERW 1 NDD ″ ″″ ″ ″ Hot-rolled, ERW 7 NDD* ″ ″ ″ ″ ″ Cold-rolled, ERW 4 NEE 0.002 0.010.2 0.02 0.03Al-0.04Ti Cold-rolled, ERW 11 NEE* ″ ″ ″ ″ ″ Cold-rolled,ERW 5 NFF 0.003 0.02 0.2 0.02 0.03Al-0.02Nb-0.03Ti-0.0018B Hot-rolled,ERW 1.2 NFF ″ ″ ″ ″ ″ Cold-rolled, ERW 9 {111}<110> − {110}<110> −{100}<110> − {111}<112> + Steel {332}<110> {110}<110> {223}<110>{100}<110> {554}<225> NAA 9.5 11 1.9 2.8 1.9 NAA 14 8 2.8 2 4 NAA* 1 0.51 1.5 0.5 NBB 10 9 1.5 2 2 NBB* 0.5 0.5 1 1 1 NCC 10 11 1.5 1 2.5 NCC* 10.5 0.5 0.5 1 NDD 1.5 0.3 10.5 3.5 0.8 NDD 8.5 9 2.3 1.5 2 NDD* 3 0 1 03.5 NEE 6.3 3 3 2 9 NEE* 3.5 0 1 0 4 NFF 1.9 0.4 8.9 4 1 NFF 5.1 2.5 2.83 7 # precipitating or crystallizing during refining, solidification,hot-rolling, etc., although it-is difficult to measure the areapercentages of all the precipitates and crystals accurately by alight-optical microscope. Thus, when the area percentage of these secondphases is small and it is difficult to measure it accurately, ferriteaccounts for over 90% of the area percentage, and, in this case, thearea percentage of ferrite is shown as “over 90%”.

[0216] TABLE 7 (continued from Table 6) Kind of steel sheet and seamwelding Steel C Si Mn P Facultative elements method {111}<110> NGG 0.050.6 1 0.03 0.05Nb Hot-rolled, 2 ERW NHH 0.003 0.1 0.3 0.02 0.4HfCold-rolled, 9 ERW NII 0.0015 0.05 0.07 0.03 0.3Ta Hot-rolled, 2.5 ERWNJJ 0.002 0.02 0.1 0.02 1.3Cu-0.6Ni Hot-rolled, 2.7 ERW NJJ ″ ″ ″ ″ ″Hot-rolled, 2.5 ERW NJJ ″ ″ ″ ″ ″ Cold-rolled, 6 ERW NKK 0.04 0.5 1.50.02 0.05Ti-0.0005Ca-0.03Al Hot-rolled, 2 ERW NLL 0.05 0.6 0.8 0.020.05Ti-0.0025Mg-0.03Al Hot-rolled, 2.2 ERW NMM 0.002 0.1 0.3 0.010.05Ti-0.0030Mg-0.01Al Cold-rolled, 10 ERW CNAA 0.45 0.2 0.2 0.01Hot-rolled, 1 ERW CNBB 0.05 0.6 0.8 0.02 1.0Nb Hot-rolled, 0.5 ERW CNCC0.002 0.02 0.2 0.01 0.05Nb-0.05Ti-0.07B Cold-rolled, 1.4 ERW {111}<110>− {110}<110> − {100}<110> − {111}<112> + Steel {332}<110> {110}<110>{223}<110> {100}<110> {554}<225> NGG 5.2 3 3.1 1 0.7 NHH 5.6 3.5 2.7 2.54.8 NII 6 3.5 3.4 2 0.6 NJJ 2.5 0.5 8.2 5 0.3 NJJ 7 5 2 0.5 2 NJJ 5 3.51.5 0.5 5 NKK 5.5 4.5 1.8 0.4 0.7 NLL 6 4 2 0.5 0.7 NMM 6 2.5 2.5 2 8CNAA 0.5 0.4 10 8 0.5 CNBB 0.2 0.3 11 7 0.5 CNCC 1.5 2.5 7.5 4.5 0.5 #it is difficult to measure the area percentages of all the precipitatesand crystals accurately by a light-optical microscope. Thus, when thearea percentage of these second phases is small and it is difficult tomeasure it accurately, ferrite accounts for over 90% of the areapercentage, and, in this case, the area percentage of ferrite is shownas “over 90%”.

[0217] TABLE 8 Average Temperature Heating Area aspect of heattemperature Finish Average Standard percentage ratio of treatment beforetemperature n-value in ferrite grain deviation of of ferrite ferriteafter pipe diameter of diameter longitudinal Steel size/μm grain size/μmgrains* grains forming/° C. reduction/° C. reduction/° C. direction NAA12 4.5 Over 90% 2.1 980 750 0.14 NAA 40 18 Over 90% 5 800 650 0.11 NAA*15 5 Over 90% 1.3 650 0.16 NBB 15 5 Over 90% 2.4 980 730 0.14 NBB* 15 5Over 90% 1.1 675 0.17 NCC 17 6 Over 90% 3 950 735 0.16 NCC* 25 8 Over90% 1.4 700 0.17 NDD 20 5 Over 90% 5.6 750 640 0.11 NDD 22 9 Over 90% 3950 750 0.16 NDD* 25 9 Over 90% 1.5 650 0.17 NEE 25 9.3 Over 90% 3.5 900750 0.17 NEE* 27 9 Over 90% 1.5 650 0.17 NFF 15 5 Over 90% 2.7 750 6000.11 NFF 24 7 Over 90% 2.9 900 730 0.15 Maximum diameter expansionn-value in r-value in ratio circumferential longitudinal when εφ/ Steeldirection direction εθ = 0.5 NAA 0.13 2.5 1.48 Invented steel NAA 0.091.8 1.31 Invented steel NAA* 0.15 0.9 1.3 Invented steel NBB 0.13 3.11.55 Invented steel NBB* 0.16 0.9 1.3 Invented steel NCC 0.15 3.8 1.59Invented steel NCC* 0.17 1.2 1.38 Invented steel NDD 0.1 0.4 1.08Comparative steel NDD 0.14 3.2 1.53 Invented steel NDD* 0.17 1.3 1.4Invented steel NEE 0.15 2.3 1.46 Invented steel NEE* 0.17 1.8 1.4Invented steel NFF 0.1 0.5 1.1 Comparative steel NFF 0.12 2 1.43Invented steel # it is difficult to measure the area percentages of allthe precipitates and crystals accurately by a light-optical microscope.Thus, when the area percentage of these second phases is small and it isdifficult to measure it accurately, ferrite accounts for over 90% of thearea percentage, and, in this case, the area percentage of ferrite isshown as “over 90%”.

[0218] TABLE 9 (continued from Table 8) Average Temperature Heating Areaaspect of heat temperature Finish Average Standard percentage ratio oftreatment before temperature n-value in ferrite grain deviation of offerrite ferrite after pipe diameter of diameter longitudinal Steelsize/μm grain size/μm grains* grains forming/° C. reduction/° C.reduction/° C. direction NGG 14 5 84% 2.3 950 840 0.12 NHH 20 4 Over 90%2.1 900 750 0.13 NII 15 5 Over 90% 2.5 930 800 0.13 NJJ 20 6 Over 90%2.8 830 630 0.1 NJJ 27 8 Over 90% 2.4 980 750 0.13 NJJ 25 6 Over 90% 2.2980 750 0.13 NKK 13 4 Over 90% 1.9 910 770 0.11 NLL 10 4 Over 90% 1.9920 780 0.11 NMM 20 7 Over 90% 2.9 900 750 0.16 CNAA Not measurable 930800 0.05 CNBB Not measurable 950 830 0.06 CNCC 23 6 Over 90% 3.5 800 6000.1 Maximum diameter expansion n-value in r-value in ratiocircumferential longitudinal when εφ/ Steel direction direction εθ = 0.5NGG 0.11 1.9 1.39 Invented steel NHH 0.12 2.1 1.4 Invented steel NII0.11 2 1.39 Invented steel NJJ 0.08 0.7 1.18 Comparative steel NJJ 0.122.1 1.4 Invented steel NJJ 0.12 2.2 1.4 Invented steel NKK 0.1 2.3 1.42Invented steel NLL 0.09 2.2 1.4 Invented steel NMM 0.14 2.3 1.44Invented steel CNAA 0.04 0.8 1.05 Comparative steel CNBB 0.05 0.7 1.05Comparative steel CNCC 0.08 0.9 1.1 Comparative steel # the precipitatesand crystals accurately by a light-optical microscope. Thus, when thearea percentage of these second phases is small and it is difficult tomeasure it accurately, ferrite accounts for over 90% of the areapercentage, and, in this case, the area percentage of ferrite is shownas “over 90%”.

Example 3

[0219] The steels of the chemical compositions shown in Tables 10 and 11were rolled into hot-rolled and cold rolled steel sheets 2.2 mm inthickness under the same conditions as in Example 1. The steel sheetswere formed into steel pipes 108 or 89.1 mm in outer diameter by TIG,laser or electric resistance welding, then heated and subjected todiameter reduction to obtain high strength steel pipes 63.5 to 25 mm inouter diameter.

[0220] Hydroforming work was applied to the steel pipes finally obtaineduntil they burst. Then the diameter expansion ratio at which the ratioρ=εφ/εθ of the strain εφ in the longitudinal direction of the pipe andthe strain εθ in the circumferential direction near the fracture or inthe portion of the maximum wall thickness reduction was −0.1 to −0.2(the value was negative as the wall thickness decreased) was calculated,and this diameter expansion ratio was used as an indicator of theformability in hydroforming for the evaluation of the product pipes.

[0221] X-ray analysis was carried out on flat test pieces prepared bycutting out arc section test pieces from the steel pipes and thenpressing them. The relative X-ray strength of the test pieces wasobtained through the comparison with the X-ray strength of a randomcrystal.

[0222] Tables 12 and 13 show, for each steel, the n-values in thelongitudinal and circumferential directions, the r-values in thelongitudinal direction, the ratios of the X-ray strength in differentorientation components and the maximum diameter expansion ratios(=maximum diameter at the time of burst/initial diameter) until thesteel pipes burst at the hydroforming.

[0223] In invented steels A to O, the n-value(s) in the longitudinaland/or circumferential directions was/were 0.18 or more and the r-valuein the longitudinal direction was less than 2.2 except for steel A whichwas formed into pipes by laser welding.

[0224] Further, in the invented steels, the average for the ratios ofthe X-ray strength in the orientation component group of {110}<110> to{111}<110> to random X-ray diffraction strength was 1.5 or more and therelative X-ray strength in the orientation component of {110}<110> was5.0 or less and, moreover, in some of them, the relative X-ray strengthin the orientation component of {111}<110> was 3.0 or more. As a result,a good diameter expansion ratio over 1.30 was obtained in them.

[0225] In high-C steel CA, high-Mg steel CB, high-Nb steel CC, high-Bsteel CE and high-Cr steel CF, in contrast, n-value was low in both thelongitudinal and circumferential directions and the diameter expansionratio was also low. These steels, except for steel CE, showed low ratiosof the X-ray strength in the orientation components {110}<110> and/or{111}<110> and the orientation component group of {110}<110> to{111}<110> to random X-ray diffraction strength, and the diameterexpansion ratio was lower still. Aside from the above, weld defectsoccurred during the pipe forming of high-P steel CD and high-Ca+Remsteel CG, demonstrating the difficulty in the pipe forming by a massproduction facility. TABLE 10 Steel C Si S Mn Al N Zr Mg Ti V Nb P B CrCu A 0.05 0.2 0.005 0.4 0.02 0.002 0.005 B 0.048 0.05 0.005 0.75 0.050.0045 0.02 C 0.002 0.04 0.003 0.1 0.02 0.0025 0.09 D 0.002 0.05 0.0060.4 0.03 0.0026 0.0011 0.06 0.01 E 0.0032 0.03 0.004 0.7 0.045 0.00290.02 0.02 0.05 0.0008 F 0.13 0.05 0.005 0.84 0.03 0.0023 G 0.035 0.40.004 1.4 0.02 0.0061 0.16 0.03 H 0.08 0.2 0.004 1.2 0.03 0.0036 0.070.03 I 0.0025 0.05 0.005 0.25 0.04 0.0032 0.04 0.04 0.9 J 0.005 1 0.0030.7 0.03 0.0035 0.01 0.02 0.02 0.2 K 0.11 0.2 0.002 1.4 0.04 0.003 0.047L 0.05 1.8 0.003 1.5 0.05 0.0036 M 0.17 1.3 0.003 1.2 0.03 0.0032 0.03 N0.05 1.5 0.002 1.1 0.04 0.0025 0.08 0.02 O 0.09 1 0.003 0.9 0.03 0.00310.01 0.04 0.03 Steel Ni Mo Co W Ca Rem A Invented steel B Invented steelC Invented steel D Invented steel E Invented steel F Invented steel GInvented steel H Invented steel I 0.3 Invented steel J 0.1 0.1 Inventedsteel K Invented steel L 0.001 0.0002 Invented steel M 0.3 Inventedsteel N Invented steel O Invented steel

[0226] TABLE 11 (continued from Table 10) Steel C Si S Mn Al N Zr Mg TiV Nb P B Cr CA 0.47 0.2 0.003 0.9 0.03 0.0025 0.01 CB 0.002 0.05 0.0020.1 0.005 0.0035 0.6 0.05 CC 0.15 0.05 0.003 0.8 0.04 0.0025 1.9 0.02 CD0.12 0.05 0.009 1.4 0.05 0.003 0.08 0.35 CE 0.0025 0.05 0.008 1.2 0.030.003 0.02 0.05 0.03 0.09 CF 0.05 0.1 0.01 1 0.03 0.007 0.03 9.1 CG 0.050.6 0.003 0.7 0.1 0.006 0.02 Steel Cu Ni Mo Co W Ca Rem CA Comparativesteel: C outside range CB Comparative steel: Mg outside range CCComparative steel: Nb outside range CD Comparative steel: P outsiderange CE Comparative steel: B outside range CF 1.2 Comparative steel:Gr, Mo outside range CG 0.07 0.46 Comparative steel: Ca, REM outsiderange

[0227] TABLE 12 Average relative X-ray strength in orientation RelativeRelative Seam component X-ray X-ray welding group of strength instrength in method for n-value in n-value in r-value in {110}<110>orientation orientation pipe longitudinal circumferential longitudinal −component of component of Steel forming direction direction direction{111}<110> {110}<110> {111}<110> A ERW 0.26 0.24 1.3 3 2.5 2 A Laser0.18 0.16 2.3 2.5 2.9 2 B ERW 0.18 0.19 2.1 4 1 5.6 C Laser 0.2 0.19 1.53 0.5 3.5 D Laser 0.18 0.19 1.3 3 0 3.5 E Laser 0.22 0.2 1.2 3.5 0 4 FERW 0.23 0.21 1.3 2 2 1.5 G ERW 0.18 0.17 1 2 1.5 2 H ERW 0.2 0.18 1.52.5 2.5 2.5 I Laser 0.19 0.19 1.4 3 0.5 3.5 J TIG 0.2 0.18 1.2 2.5 0 3 KERW 0.21 0.18 1.9 3.5 2.8 3.2 L ERW 0.23 0.2 2 3.5 2.8 2.5 M Laser 0.210.2 1.2 2.5 2 3 N ERW 0.2 0.19 1.2 2.5 2.5 2.5 O ERW 0.21 0.19 1.3 2.5 23 Diameter expansion Percentage ratio at Area Aspect of grains burstingpercentage ratio 0.1-200 Steel by HF of ferrite of ferrite μm in size(%) A 1.45 Over 90% 2.3 100 Invented steel-hot A 1.38 Over 90% 2.5 100Invented steel-hot B 1.45 Over 90% 1.6 100 Invented steel-cold C 1.38Over 90% 1.5 100 Invented steel-cold D 1.35 Over 90% 1.4 100 Inventedsteel-cold E 1.41 Over 90% 1.4 100 Invented steel-cold F 1.4 Over 90%1.6 100 Invented steel-hot G 1.34 Over 90% 1.5 100 Invented steel-hot H1.43 87% 1.7 100 Invented steel-hot I 1.39 Over 90% 1.3 100 Inventedsteel-cold J 1.35 Over 90% 1.4 100 Invented steel-hot K 1.4 84% 1.9 100Invented steel-hot L 1.44 Over 90% 1.5 100 Invented steel-hot M 1.41 82%1.8 100 Invented steel-cold N 1.41 Over 90% 2.3 100 Invented steel-hot O1.42 Over 90% 1.5 100 Invented steel-hot # of all the precipitates andcrystals accurately by a light-optical microscope. Thus, when the areapercentage of these second phases is small and it is difficult tomeasure it accurately, ferrite accounts for over 90% of the areapercentage, and, in this case, the area percentage of ferrite is shownas “over 90%”.

[0228] TABLE 13 (continued from Table 12) Average relative X-raystrength in orientation Relative X-ray Relative X-ray Diameter Seamwelding component group of strength in strength in expansion method forn-value in n-value in r-value in {110}<110> orientation orientationratio at pipe longitudinal circumferential longitudinal − component ofcomponent of bursting Steel forming direction direction direction{111}<110> {110}<110> {111}<110> by HF CA ERW 0.11 0.11 1 1.5 0.5 1 1.04CB Laser 0.11 0.1 1 1 1 1 1.03 CC Laser 0.1 0.09 0.9 1 1 1 1.03 CD ERWNot tested owing to cracks and weld defects during seam welding CE Laser0.1 0.11 1 1.5 0.5 1.4 1.1 CF TIG 0.09 0.1 0.8 0.5 0.5 0.5 1.03 CG ERWNot tested owing to cracks and weld defects during seam welding AreaPercentage of grains percentage Aspect ratio 0.1-200 Steel of ferrite offerrite μm in size (%) CA Over 90% 1.5 100 Comparative steel-cold; Coutside range CB Not measurable because of Comparative steel-cold; toofine grains Mg outside range CC Not measurable because of Comparativesteel-hot; too fine grains Nb outside range CD Comparative steel-cold; Poutside range CE Over 90% 4.2 100 Comparative steel-cold; B outsiderange CF Aspect ratio and Comparative steel-hot; size distribution Cr,Mo outside range of ferrite grains not measurable owing to less than 10%of ferrite amount, over 90% being martensite or bainite. CG Comparativesteel-hot; Ca, REM outside range # during refining, solidification,hot-rolling, etc., although it is difficult to measure the areapercentages of all the precipitates and crystals accurately by alight-optical microscope. Thus, when the area percentage of these secondphases is small and it is difficult to measure it accurately, ferriteaccounts for over 90% of the area percentage, and, in this case, thearea percentage of ferrite is shown as “over 90%”.

Example 4

[0229] Among the steels of the chemical compositions shown in Tables 10and 11, steels A, F, H, K and L were refined on a laboratory scale,heated to 1,200° C., hot-rolled into steel sheets 2.2 mm in thickness ata finish rolling temperature from 10° C. below the Ar₃ transformationpoint, which is determined by the chemical composition and cooling rateof the steel, to less than 120° C. above the Ar₃ transformation point(roughly 900° C.), and the steel sheets thus produced were used as thematerials for pipe forming.

[0230] The steel sheets were formed, in the cold, into steel pipes 108or 89.1 mm in outer diameter by electric resistance welding. Thereafter,the steel pipes were subjected to diameter reduction work to obtain highstrength steel pipes 63.55 to 25 mm in outer diameter at the heatingtemperatures and diameter reduction temperatures shown in Table 14.

[0231] Hydroforming work was applied to the steel pipes finally obtaineduntil they burst. Then, the diameter expansion ratio at which the ratioρ=εφ/εθ of the strain εφ in the longitudinal direction of the pipes andthe strain εθ in the circumferential direction near the fracture or inthe portion of the maximum wall thickness reduction was −0.1 to −0.2(the value was negative as the wall thickness decreased) was calculated,and this diameter expansion ratio was used as an indicator of theformability in hydroforming for the evaluation of the product pipes.

[0232] Table 14 shows the characteristics of the steels. In the steelssatisfying the production conditions specified in claim 34, the n-valuesin the longitudinal and circumferential directions were 0.18 or more andthe r-value in the longitudinal direction was less than 2.2.

[0233] Further, in these steels, the average for the ratios of the X-raystrength in the orientation component group of {110}<110> to {111}<110>to random X-ray diffraction strength was 1.5 or more and the relativeX-ray strength in the orientation component of {110}<110> was 5.0 orless and, moreover, in some of them, the relative X-ray strength in theorientation component of {111}<110> was 3.0 or more. As a result, a gooddiameter expansion ratio over 1.30 was obtained in these steels.

[0234] In contrast, in the steels not satisfying the productionconditions specified in claim 34, n-value was low in both thelongitudinal and circumferential directions. However, since the steelssatisfied any one of claims 1, 9, 10, 11 and 19, their diameterexpansion ratios were comparatively good, roughly 1.25 or higher, if notvery high in the above forming mode. The steels which underwent thediameter reduction work at a high diameter reduction ratio of 77% brokeduring the work. TABLE 14 Heating Average relative temperature X-raystrength for diameter Finish in orientation reduction temperaturecomponent group work after of diameter Diameter n-value in n-value inr-value in of {110}<110> pipe forming reduction reduction longitudinalcircumferential longitudinal − Steel /° C work/° C. ratio/% directiondirection direction {111}110> A 980 800 29 0.26 0.24 1.3 3 980 650 580.16 0.17 2.5 3.5 980 700 77 F 950 760 29 0.23 0.21 1.3 2 950 650 580.12 0.14 2.6 4 870 800 29 0.24 0.22 1 2.5 H 950 770 29 0.2 0.18 1.5 2.5950 700 77 K 950 780 29 0.21 0.18 1.9 3.5 950 650 58 0.1 0.09 2.3 4 L980 840 29 0.23 0.2 2 3.5 980 650 58 0.14 0.13 2.4 4 Relative X-rayRelative X-ray strength in strength in orientation orientation componentof component of Diameter expansion Steel {110}<110> {111}<110> ratio atHF A 2.5 2 1.45 Invented example (according to claim 34) 5 3.5 1.26Invented example Broken at diameter Comparative example reduction F 21.5 1.4 Invented example (according to claim 34) 5.5 3 1.25 Inventedexample 1 1 1.42 Invented example (according to claim 34) H 2.5 2.5 1.43Invented example (according to claim 34) Broken at diameter Comparativeexample reduction K 2.8 3.2 1.4 Invented example (according to claim 34)5.5 3.2 1.26 Invented example L 2.8 2.5 1.44 Invented example (accordingto claim 34) 4 3 1.26 Invented example

Industrial Applicability

[0235] The present invention makes it possible to produce a highstrength steel pipe excellent in formability in hydroforming and similarforming techniques by identifying the texture of a steel materialexcellent in formability in hydroforming and similar forming techniquesand a method of controlling the texture and by specifying the textureand the controlling method.

1. A steel pipe excellent in formability characterized by: containing,in mass, C: 0.0005 to 0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0%, withthe balance consisting of Fe and unavoidable impurities; and the averagefor the ratios of the X-ray strength in the orientation component groupof {110}<110> to {111}<110> to random X-ray diffraction strength on aplane at the wall thickness center being 2.0 or more and/or the ratio ofthe X-ray strength in the orientation component of {110}<110> to randomX-ray diffraction strength on a plane at the wall thickness center being3.0 or more.
 2. A steel pipe excellent in formability according to claim1, characterized by further containing, in the steel, one or more of Al,Zr and Mg at 0.0001 to 0.5 mass % in total.
 3. A steel pipe excellent informability according to claim 1 or 2, characterized by furthercontaining, in the steel, one or more of Ti, V and Nb at 0.001 to 0.5mass % in total.
 4. A steel pipe excellent in formability according toany one of claims 1 to 3, characterized by further containing P at 0.001to 0.20 mass % in the steel.
 5. A steel pipe excellent in formabilityaccording to any one of claims 1 to 4, characterized by furthercontaining B at 0.0001 to 0.01 mass % in the steel.
 6. A steel pipeexcellent in formability according to any one of claims 1 to 5,characterized by further containing, in the steel, one or more of Cr,Cu, Ni, Co, W and Mo at 0.001 to 1.5 mass % in total.
 7. A steel pipeexcellent in formability according to any one of claims 1 to 6,characterized by further containing, in the steel, one or more of Ca anda rare earth element (Rem) at 0.0001 to 0.5 mass % in total.
 8. A steelpipe excellent in formability according to any one of claims 1 to 7,characterized in that: ferrite accounts for 50% or more, in terms ofarea percentage, of the metallographic structure; the crystal grain sizeof the ferrite is within the range from 0.1 to 200 μm; and the averagefor the ratios of the X-ray strength in the orientation component groupof {110}<110> to {111}<110> to random X-ray diffraction strength on aplane at the wall thickness center is 2.0 or more and/or the ratio ofthe X-ray strength in the orientation component of {110}<110> to randomX-ray diffraction strength on a plane at the wall thickness center is3.0 or more.
 9. A steel pipe excellent in formability characterized bysatisfying either one or both of the following properties: {circle over(1)} the n-value in the longitudinal direction of the pipe being 0.12 ormore, and {circle over (2)} the n-value in the circumferential directionof the pipe being 0.12 or more.
 10. A steel pipe excellent informability according to claim 9, characterized by having the propertyof the r-value in the longitudinal direction of the pipe being 1.1 ormore.
 11. A steel pipe excellent in formability characterized in thatthe texture of the steel pipe satisfies one or more of the followingconditions {circle over (1)} to {circle over (3)}. {circle over (1)} atleast one or more of the following ratios being 3.0 or more: the ratioof the X-ray strength in the orientation component of {111}<110> torandom X-ray diffraction strength on a plane at the wall thicknesscenter; the average for the ratios of the X-ray strength in theorientation component group of {110}<110> to {332}<110> to random X-raydiffraction strength on a plane at the wall thickness center; and theratio of the X-ray strength in the orientation component of {110}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter, {circle over (2)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and {circle over (3)} at least either one or bothof the following conditions being satisfied: the average for the ratiosof the X-ray strength in the orientation component group of {111}<110>to {111}<112> and {554}<225> to random X-ray diffraction strength on aplane at the wall thickness center being 2.0 or more; and the ratio ofthe X-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength on a plane at the wall thickness center being3.0 or more.
 12. A steel pipe excellent in formability according to anyone of claims 9 to 11, characterized by containing ferrite at 50% ormore in terms of area percentage and the grain size of the ferrite beingin the range from 0.1 to 200 μm.
 13. A steel pipe excellent informability according to any one of claims 9 to 12, characterized by:containing ferrite at 50% or more in terms of area percentage; the grainsize of the ferrite ranging from 1 to 200 μm; and the standard deviationof the distribution of the grain size falling within the range of ±40%of the average grain size.
 14. A steel pipe excellent in formabilityaccording to any one of claims 9 to 13, characterized by: containingferrite at 50% or more in terms of area percentage; and the average forthe aspect ratios (the ratio of the grain length in the longitudinaldirection to the grain thickness in the thickness direction) of ferritegrains being in the range from 0.5 to 3.0.
 15. A steel pipe excellent informability according to any one of claims 9 to 14, characterized bycontaining, in mass, C: 0.0005 to 0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to3.0%, P: 0.001 to 0.20%, and N: 0.0001 to 0.03%, with the balanceconsisting of Fe and unavoidable impurities.
 16. A steel pipe excellentin formability according to claim 15, characterized by furthercontaining in the steel pipe, in mass, one or more of Ti: 0.001 to 0.5%,Zr: 0.001 to 0.5% or less, Hf: 0.001 to 2.0% or less, Cr: 0.001 to 1.5%or less, Mo: 0.001 to 1.5% or less, W: 0.001 to 1.5% or less, V: 0.001to 0.5% or less, Nb: 0.001 to 0.5% or less, Ta: 0.001 to 2.0% or less,and Co: 0.001 to 1.5% or less.
 17. A steel pipe excellent in formabilityaccording to claim 15 or 16, characterized by further containing, in thesteel pipe, in mass, one or more of B: 0.0001 to 0.01%, Ni 0.001 to1.5%, and Cu: 0.001 to 1.5%.
 18. A steel pipe excellent in formabilityaccording to any one of claims 15 to 17, characterized by furthercontaining, in the steel pipe, in mass, one or more of Al: 0.001 to0.5%, Ca: 0.0001 to 0.5%, Mg: 0.0001 to 0.5%, and Rem: 0.0001 to 0.5%.19. A method of producing a steel pipe excellent in formabilityaccording to any one of claims 1 to 18, characterized by forming amother pipe using a hot-rolled or cold-rolled steel sheet satisfying anyone or more of the following conditions {circle over (1)} to {circleover (4)} as the material sheet, then heating the mother pipe to atemperature in the range from the Ac₃ transformation point to 200° C.above the Ac₃ transformation point, and then subjecting it to diameterreduction work in the temperature range from 900 to 650° C.: {circleover (1)} at least either one or both of the following conditions beingsatisfied: the average for the ratios of the X-ray strength in theorientation component group of {110}<110> to {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center being 2.0or more; and the ratio of the X-ray strength in the orientationcomponent of {110}<110> to random X-ray diffraction strength on a planeat the wall thickness center being 3.0 or more, {circle over (2)} atleast one or more of the following ratios being 3.0 or more: the ratioof the X-ray strength in the orientation component of {111}<110> torandom X-ray diffraction strength on a plane at the wall thicknesscenter; the average for the ratios of the X-ray strength in theorientation component group of {110}<110> to {332}<110> to random X-raydiffraction strength on a plane at the wall thickness center; and theratio of the X-ray strength in the orientation component of {110}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter, {circle over (3)} at least either one or both of the followingratios being 3.0 or less: the average for the ratios of the X-raystrength in the orientation component group of {100}<110> to {223}<110>to random X-ray diffraction strength on a plane at the wall thicknesscenter; and the ratio of the X-ray strength in the orientation componentof {100}<110> to random X-ray diffraction strength on a plane at thewall thickness center, and {circle over (4)} at least either one or bothof the following conditions being satisfied: the average for the ratiosof the X-ray strength in the orientation component group of {111}<110>to {111}<112> and {554}<225> to random X-ray diffraction strength on aplane at the wall thickness center being 2.0 or more; and the ratio ofthe X-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength on a plane at the wall thickness center being3.0 or more.
 20. A method of producing a steel pipe excellent informability according to any one of claims 1 to 18, characterized byforming a mother pipe using a hot-rolled or cold-rolled steel sheetsatisfying any one or more of the following conditions {circle over (1)}to {circle over (4)} as the material sheet, and then applying heattreatment to the mother pipe at a temperature in the range from 650° C.to 200° C. above the Ac₃ transformation point: {circle over (1)} atleast either one or both of the following conditions being satisfied:the average for the ratios of the X-ray strength in the orientationcomponent group of {110}<110> to {111}<110> to random X-ray diffractionstrength on a plane at the wall thickness center being 2.0 or more; andthe ratio of the X-ray strength in the orientation component of{110}<110> to random X-ray diffraction strength on a plane at the wallthickness center being 3.0 or more, {circle over (2)} at least one ormore of the following ratios being 3.0 or more: the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center; theaverage for the ratios of the X-ray strength in the orientationcomponent group of {110}<110> to {332}<110> to random X-ray diffractionstrength on a plane at the wall thickness center; and the ratio of theX-ray strength in the orientation component of {110}<110> to randomX-ray diffraction strength on a plane at the wall thickness center,{circle over (3)} at least either one or both of the following ratiosbeing 3.0 or less: the average for the ratios of the X-ray strength inthe orientation component group of {100}<110> to {223}<110> to randomX-ray diffraction strength on a plane at the wall thickness center; andthe ratio of the X-ray strength in the orientation component of{100}<110> to random X-ray diffraction strength on a plane at the wallthickness center, and {circle over (4)} at least either one or both ofthe following conditions being satisfied: the average for the ratios ofthe X-ray strength in the orientation component group of {111}<110> to{111}<112> and {554}<225> to random X-ray diffraction strength on aplane at the wall thickness center being 2.0 or more; and the ratio ofthe X-ray strength in the orientation component of {111}<110> to randomX-ray diffraction strength on a plane at the wall thickness center being1.5 or more.
 21. A steel pipe excellent in formability characterized bysatisfying either one or both of the following properties: {circle over(1)} the n-value in the longitudinal direction of the pipe being 0.18 ormore, and {circle over (2)} the n-value in the circumferential directionof the pipe being 0.18 or more.
 22. A steel pipe excellent informability according to claim 21, characterized by having the propertyof the r-value in the longitudinal direction of the pipe being 0.6 ormore but less than 2.2.
 23. A steel pipe excellent in formabilityaccording to claim 21 or 22, characterized in that the ratio of X-raystrength to random X-ray diffraction strength satisfies the followingtwo conditions: {circle over (1)} the average for the ratios of theX-ray strength in the orientation component group of {110}<110> to{111}<110> to random X-ray diffraction strength on a plane at the wallthickness center being 1.5 or more, and {circle over (2)} the ratio ofthe X-ray strength in the orientation component of {110}<110> to randomX-ray diffraction strength on a plane at the wall thickness center being5.0 or less.
 24. A steel pipe excellent in formability according to anyone of claims 21 to 23, characterized in that the ratio of the X-raystrength in the orientation component of {111}<110> to random X-raydiffraction strength on a plane at the wall thickness center is 3.0 ormore.
 25. A steel pipe excellent in formability according to any one ofclaims 21 to 24, characterized by containing ferrite at 50% or more interms of area percentage and the grain size of the ferrite being in therange from 0.1 to 200 μm.
 26. A steel pipe excellent in formabilityaccording to any one of claims 21 to 25, characterized by: containingferrite at 50% or more in terms of area percentage; and the average forthe aspect ratios (the ratio of the grain length in the longitudinaldirection to the grain thickness in the thickness direction) of ferritegrains being in the range from 0.5 to 3.0.
 27. A steel pipe excellent informability according to any one of claims 21 to 26, characterized bycontaining, in mass, C: 0.0005 to 0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to3.0%, and N: 0.0001 to 0.03%, with the balance consisting of Fe andunavoidable impurities.
 28. A steel pipe excellent in formabilityaccording to any one of claims 21 to 27, characterized by furthercontaining, in the steel pipe, one or more of Al, Zr and Mg at 0.0001 to0.5 mass % in total.
 29. A steel pipe excellent in formability accordingto any one of claims 21 to 28, characterized by further containing, inthe steel pipe, one or more of Ti, V and Nb at 0.001 to 0.5 mass % intotal.
 30. A steel pipe excellent in formability according to any one ofclaims 21 to 29, characterized by further containing P at 0.001 to 0.20mass %, in the steel pipe.
 31. A steel pipe excellent in formabilityaccording to any one of claims 21 to 30, characterized by furthercontaining B at 0.0001 to 0.01 mass %, in the steel pipe.
 32. A steelpipe excellent in formability according to any one of claims 21 to 31,characterized by further containing, in the steel pipe, one or more ofCr, Cu, Ni, Co, W and Mo by 0.001 to 5.0 mass % in total.
 33. A steelpipe excellent in formability according to any one of claims 21 to 32,characterized by further containing, in the steel pipe, one or more ofCa and a rare earth element (Rem) by 0.0001 to 0.5 mass % in total. 34.A method of producing a steel pipe excellent in formability according toany one of claims 21 to 33, characterized by forming a mother pipe, thenheating it to a temperature in the range from 50° C. below the Ac₃transformation point to 200° C. above the Ac₃ transformation point, andthen subjecting it to diameter reduction work in the temperature rangefrom 650 to 900° C. at a diameter reduction ratio of 10 to 40%.